Notes on the Anatomy of Diadromus (Thyraeella) Collaris Grav. (Ichneumonidae, Hymenoptera).

Transactions and Proceedings of the Royal Society of New Zealand, Volume 74, 1944-45, Page 154

Notes on the Anatomy of Diadromus (Thyraeella) Collaris Grav. (Ichneumonidae, Hymenoptera). By B. B. Given, M.Sc., Entomology Division, Plant Research Bureau, Nelson. [Read before the Nelson Philosophical Society, August 9, 1943; received by the Editor, May 3, 1944; issued separately, September, 1944.] Introduction. The Ichneumon, Diadromus collaris was introduced into New Zealand from Western Europe during the summer and autumn of 1937–38, as a pupal parasite of the diamond-back moth, Plutella maculipennis. It was successfully established in New Zealand, and has become a very active agent in the control of Plutella. The extreme ease with which this parasite can be bred and handled in the laboratory, its hardiness, the short duration of its life-cycle, and the longevity of the adults, all make it an ideal subject for study, both morphologically and ecologically. The lack of structural information on the Ichneumonidae is surprising, especially when we consider the taxonomic difficulties of the group, which cannot be overcome except by thorough anatomical knowledge. The high degree of specialisation of function, and the parallel diversity of anatomical structure, give us a most absorbing and in the realm of insect physiology, a most instructive field for research. The following notes describe the structure of a typical Ichneumon, and have little claim to originality. However, it is hoped that the information contained will be of value in helping to form the basis of a better understanding of the structure and function of a very interesting and complicated group of insects. The Thorax (Plate 16, Figs. 1 and 2). (a) The Prothorax. The prothorax consists of a pronotum (pr.) and a prosternum (prs.). The propleura have become fused with the lateral cervicals (ltc.). The pronotum is a large sclerite, narrow in front, but curving around and back to form two large triangular plates, extending around the edge of the episternum to a point just above the coxae (cx.) of the prothoracic legs. The prosternum is a small trapezoid plate lying between the posterior ends of the lateral cervicals, and obscured by the bulbous procoxae. (b) The Mesothorax. Dorsally, the mesothorax consists of a Prescutum (pres.) and scutum (so.) together forming the most conspicuous part of the thorax, and the scutellum (scl.) and parascutellum (pscl.). The scutum and prescutum form a large, rounded, somewhat trilobed plate which anteriorly meets the transverse arch of the pronotum.

The anterior lobe is the prescutum, and the lateral lobes, separated from it by the parapsidal sutures, are the parapsides. It is at the base of these parapsides that the tegulae and wing attachments are located. Separated from the scutum by the scutellar fovea (scv.), is the scutellum and parascutellum. The scutellum curves forward laterally to embrace the posterior portion of the scutum. It is a comparatively narrow, arched sclerite, and in Diadromus, is not distinctly divided from the parascutellum, which consists of two triangular plates extending to the bases of the wings, and between them. The mesosternal and mesopleural elements are fused to form the largest sclerite in the thorax. Below, this plate bridges the gap between the fore and mid legs, and is divided by ridges to a definite mesosternum (meso.), episternum (eps.), and anterior angled prepectus (prpt.). The pleural suture (pis.) is represented by a strong ridge, leading up to the processes of the hind wing. The upper portion of this pleural plate is the mesepisternum (mps.), while the lower and more posterior portion is the mesepimeron (mpm.). Between the prepectus and the mesepisternum is the prepectal carina (ptc.). In D. collaris, the line of demarkation between the mesosternum and the mesepimeron is also somewhat carinate. (c) The Metathorax. In the metathorax of Diadromus, the preseutum, scutum, scutellum and postscutellum, are not clearly defined. The presoutum is hidden by, and merges with, the mesopostscutellar region, and is not seen on casual observation. The scutum merges from the prescutum, and is seen as a narrow, arched, transverse band. The metascutellum (mts.) is fused with this element, and together they form the major portion of the metanotum. The metapostscutellum (mtps.) is not in any way differentiated. The fusion of the anepisternum (anep.) and anepimeron (apm.) is complete, and these fused plates curve back below the first abdominal spiracle to unite with the large plate formed by the fusion of the katepisternum (kts.) and katepimeron (ktm.). The Propodium (Plate 16, Figs. 1 and 2). The propodium is morphologically closely related to the thorax, and on casual observation might easily be mistaken for the metanotum. It consists of the first abdominal tergite. The corresponding pleural and sternal elements have become completely atrophied. Dorsally, it slopes away from the metanotum to its horseshoe-shaped posterior extremity, where it articulates with the petiole. The propodium is divided by carinae into three median and five lateral areas. The median dorsal areas are the basareola (bas.) in front, the hexagonal areola (ar.) in the centre, and the petiolarea (pet.) extending back to the petiolar articulation. Laterally, the two anterior and uppermost areas are the external or first lateral area (lat.1) in front, and the area dentipara, external median, or second lateral (lat.2) area behind it, extending back from the second median area or areola. The small antero-lateral area in front of the spiracle is the spiracular area (spa.), and the long, narrow portion behind the spiracle is divided into the lateral, middle pleural, or second pleural (mp.) area in front and the internal, middle apical, or third lateral (lat.3) area behind.

Wing-Coupling (Plate 16, Figs. 3, 4, and 5). The wing-coupling apparatus is on the typical hymenopterous plan. The trailing edge of the fore-wing is bent over in two areas to form folds (fd.), and into these folds fit hamuli (hm.) which project backward from the leading edge of the hind wing. The folds on the fore-wing occur towards each end of the hyaline post-anal area, and are slightly stiffened by extra chitinisation. The hamuli of the hind wing are placed in two corresponding sets. The proximal set consists of only one hamulus in the form of a very stout bristle with a hooked, bifid tip. Just proximal to this hamulus are several large, straight bristles which apparently serve to keep the folded portion of the fore-wing firmly pressed against the hamulus proper. In this proximal coupling the fore-wing is not closely applied to the hind-wing, the hamulus projecting straight and vertical before it forks, and the wings being held apart by the set of bristles previously mentioned. The distal coupling is attained by the association of a fold in the fore-wing with a series of five stout hamuli on the hind-wing. These distal hamuli, unlike the proximal ones, curve backwards and outwards from their bases. The most proximal member of this set is the shortest and has a simplex apex. The remainder increase in length progressively and show increased development of the bifid tip from the innermost member outwards. The wing membrane in the proximity of these hamuli bears only short papillate hairs, indicating that the overlap of the fore-wing is in this region in close apposition to the dorsal surface of the hind-wing. The Articulation of the Wings (Plate 16, Figs. 6 and 7). The wing articulates with the thprax by means of a complicated set of sclerites or pteralia set in a membranous area. These sclerites are very delicate, and it is only with great difficulty that their true relationships can be determined in the smaller members of the Hymenoptera. The arrangement is rather more complex in the fore-wing than in the hind-wing, but both possess essentially similar elements. The articulation of the fore-wing is by means of a basicostal (be.) (humeral) plate on which the costa (C), radius (R), media (M) and cubitus (Cu) articulate, a group of four axillary sclerites (ax 1 — ax 4), and a median plate (mpl.). The anal vein (A) articulates on the third axillary and median plates. The basicostar is a very large, heavy sclerite, which lies between the tegula (tg.) and the four anterior veins of the wing. In Diadromus it is separated from the costal, radial, and medial bases by a comparatively large oval sclerite, while the cubital articulates with an elongate sclerite interposed between it and the oval element mentioned above. Between this “basicubital” (bel.) plate and the lower distal prolongation of the basicostal, lies the flattened median plate, which is subdivided, with a narrow subsidiary portion forming an articulation between the median proper and the upper base of the anal vein. Proximally, the basicostal plate articulates with the large and complex first axillary plate. The second axillary articulates anteriorly with a distal arm of the first axillary, and posteriorly with the base of the third axillary,

which lies at an acute angle with it. Anteriorly and distally, the third axillary articulates with the lower base of the anal vein. The fourth axillary is a very small plate lying embedded in the membrane posterior to the third axillary. In the hind-wing, the basicostal appears to be fused with the base of the anterior wing vein complex. The median is a large, thin plate articulating between the anal vein and the base of the median vein. It is more or less triangular in outline, with its acute apex forming its anterior articulation. The first axillary is three-branched, with its anterior and longest prolongation articulating with the basicostal, and its distal prolongation articulating with the base of the second axillary. The second axillary is more or less triangular, with its sides somewhat inflexed, and its base placed distally. The anterior angle articulates with the subcosta, and the lower portion articulates with both the first axillary and the elongate third axillary. The third axillary articulates proximally with both the first and second axillaries, and distally with the base of the anal vein. As in the fore-wing, the fourth axillary is very small, and lies in the membrane posterior to the main complex. Mouth-parts (Plate 17, Fig. 8). The labrum (1m.) is small, and attached to the under surface of the clypeus (clp.) near its centre. It is armed anteriorly with sensory hairs, and posteriorly with comb-like bristled plates. Below it is a membranous epipharynx (ephy.). The mandibles (md.) are fairly slender, curved, and terminate in two blunt teeth. The outer tooth is somewhat the longer of the two, and the bifid nature of the tip is continued basally in the form of a- groove. The articulation of the mandibles is by means of a pair of ball-and-socket joints. In the dorsal articulation, the socket is on the mandible, while the condyle is on the pleurostoma. The ventral articulation consists of a condyle on the mandible itself, and a deep socket in the base of the gena. There are two rows of long sensory hairs on the inner face of each mandible, two large curved hairs on the outer side of the outer tooth, and several similar hairs just above the condyle. There is also a row of plocoid sensilla on the inner side of the mandible, and three or four stout hairs above the lower condyle. The maxillae are well developed, and well supplied with sensory hairs. The cardo (cd.) is not strongly chitinised, and is devoid of sensory apparatus. The upper end is broad and somewhat flattened, tapering to a “waist” behind. Behind this constriction it bifurcates to form inner and outer prolongations to which muscles are attached. The stipes (st.) is considerably larger than the cardo, and articulates with it on a broadened base. Distally, the stipes articulates on its outer side with the maxillary palp (mx. plp.), and on its inner side with the galea and lacinia. Running diagonally from the inner side at the base, to the outer side at the socket of the palp, is a distinct carina. The stipes has only a few sensory hairs, nearly all being placed on the outer basal portion. The maxillary palp is five-jointed, and richly supplied with sensory hairs. The basal segment is elongate and slender, and articulates at an angle with the second segment, which is more or less straight in outline on its outer side, but much

swollen on its inner side. The third segment is slightly longer and less bulbous, while the last two segments are slender and about the same length as the first segment. Sensory hairs and setae are present on the distal portion of the first segment, and occur densely over the entire surface of the other segments. The galea (ga.) is flattened and spathulate. Around its outer edge are setal sensilla, while over its inner edge and part of the surface, are very fine hairs. The lacinia (lc.) is much smaller, and fits into the curved base of the galea. It is entirely covered with fine hairs. The labial assemblage is comparatively small, the entire length of the prementum including the ligula being only slightly greater than that of the stipes. The submentum (smt.) is membranous, and carries no sensory hairs. The prementum (prmt.) is pyriform, heavily chitinised, and supplied with scattered sensory hairs and placoid sensilla. It is very strongly inflexed at the margins. At its distal end the glossae (gl.) and paraglossae (pgl.) articulate, and just below these, the labial palps (lb. plp.) arise from definite palpigers (plg.). These palps are four-jointed structures with a similar supply of sensilla to those present on the maxillary palps. In the labial palps, the middle two segments are squat and pyriform. The glossa is a large flat bilobed structure with numerous parallel grooves and ridges in its surface. It contains several types of sensilla, including the symmetrically placed, almost straight chaetae projecting anteriorly, and an anterior row of basiconic sensilla. The apical margin is in the form of a comb, with straight, close bristles all of uniform length. Paraglossae occur basally at each side of the glossa, and these pad-like structures are covered dorsally with short, stout, curved sensilla. The papillate hypopharynx at the base of the glossa marks the opening of the salivery duct. Alimentary System (Plates 17 and 18). The mouth opens into the pharynx (ph.) which lies vertically above it. The pharynx (Fig. 10) is lined with thick chitin and is shaped more or less like a D lying with its flat side ventral. The floor of the pharynx is in the form of a chitinous plate slightly raised in the middle. The lateral angles are much folded, and very flexible, allowing the roof and floor to be pressed together by muscular contraction. Strong muscles are inserted in this chitinous lining both dorsally and ventrally. Dorsally these muscles are mainly longitudinal, while ventrally there are strong transverse bands. The epithelium of the fore-gut is homologous with the hypodermis, and in the vicinity of the pharynx is fairly typical of it. In the oesophagus proper (oe.) which extends from the pharynx back to the posterior end of the petiole, the layers take on a more typical arrangement (Fig. 11). The cuticle is very thin, and raised to form innumerable small lobules and papillae. These contain prolongations of the underlying epithelium, the cells of which are more or less cubical. These cells have medium-sized nuclei, and do not appear to be markedly glandular or secretory in function. Outside this epithelium is a basement membrane, and outside this again are scattered longitudinal muscle fibres overlain by circular fibres, both bearing fine striations.

The crop (cr.) appears to be structurally a mere enlargement of the oesophagous, and consists of precisely the same elements. Towards its posterior end, however, near the proventriculus (pvent.), it becomes remarkably thin-walled, and muscle development is greatly reduced. The proventriculus or gizzard (pvent.) in Diadromus is very highly differentiated from the rest of the fore-gut, and projects into the mid-gut in the form of a definite oesophageal invagination (Fig. 15). In the proventriculus proper the muscle layers are greatly increased. The circular layer is particularly strong in its development, and forms a distinct sphincter surrounding the entire organ. From about the middle of the proventriculus, the epithelium increases in development until towards the posterior end the cells are definitely columnar, and form an internal circular ridge, falling away behind to the large columnar cells of the oesophageal invagination. The invagination is covered with epithelial cells on both sides, and may be likened to a rubber tube, the edge of which has been turned back on itself. However, it contains no muscle layers, and the inter-epithelial space appears to be in the form of a blood sinus (bis.). It is interesting to note that the proventriculus is lined outside with a definite columnar epithelium, but whether this represents the extent of the folding back of the epithelium of the invagination, or whether it is mesodermal in origin, is difficult to determine. At the junction of the oesophageal invagination and the mid-gut, a definite peritropic groove (pgr.) is present, and it is from this point that the extremely delicate peritrophic membrane (pmb.) is formed. This membrane lines the entire mid-gut, and apparently disintegrates from its posterior extremity. The cells of the mid-gut epithelium are large and columnar. They are highly vacuolate basally, and often very finely vacuolate distally. The nuclei are very large, and usually contain two or three large aggregates of chromatin material. Distally, the cells appear to be bordered by a vertically striated layer in which lies the peritrophic membrane. Groups of regenerative cells (rgr.) appear at irregular intervals at the bases of the epithelium, and these produce new epithelial cells. The muscle layers are not obviously striated, and are only weakly developed. In the mid-gut, the muscles are in the opposite position from the fore- and hind-gut, having the longitudinal fibres outermost. The mid-gut extends to the base of the fourth post-petiolar segment, and here it narrows to a short tube from which arise the twelve malpighian tubules (mlp). The hind-gut consists of the same layers as are present in the fore-gut, but developed to a very different degree. In Diadromus, the ileum (il.) and colon (cln.) are small and nearly all the proctodaeumal region of the gut is taken up by the large pyriform rectum (rect.). The ileum (Fig. 9) and colon are not differentiated, and there is no apparent difference in the elements constituting their wall. They are lined throughout with cuticle, and beneath this lies a cubical epithelium. The muscle layers (consisting of an inner circular layer, and an outer longitudinal layer) are weakly developed, and not visibly striated. The nuclei of the epithelial cells are small.

The rectum (Fig. 16) is very large, and narrows to the anus posteriorly. It is lined with cubical epithelium, and has, besides the typical muscle layers, a secondary inner circular layer of simple, unstriated fibres. The outer circular layer is strongly developed and striated. The rectal glands (Fig. 14), although six in number, are not arranged in such a manner as to give the rectum any semblance of a hexagonal cross section as is often the case. They are roughly arranged in two sets of three, one set behind, and alternating with the anterior set. These rectal glands consist of large, tapering columnar cells, overlaid as is the entire hind-gut, with a chitinous cuticle. Basally these cells contain very large vacuoles, which contain a loose granular reticulum. The entire cytoplasm is reticulate except at the internal margin where it is very dense, and contains several vacuoles. The nuclei are very large, and contain no large karyosomes. Beneath each group of columnar cells is a basal “pad” (bp.) of intersticial cells. Posteriorly, the chitinous lining of the rectum is very greatly thickened. There is a rectal valve (rect. v.) which appears actually to consist of two valves with a chamber between. The posterior portion of the rectum, or anus proper, which lies behind the anterior valve, is without longitudinal muscle fibres, but is surrounded by very strongly, developed circular muscles. According to Wigglesworth (1939), the hind-gut, and particularly the rectal glands, have a water-absorbing function, and cause the removal of water from the faecal matter, with a corresponding replenishing of body-moisture. Reproductive System (Plates 19 and 20). The reproductive system of parasitic Hymenoptera is of great interest on account of its modifications to conform with highly specialised means of oviposition. The ovipositor is a remarkable example of natural adaptation, and merits close study as regards both basic structure, and mechanical function. The male genitalia, although lacking the obvious functional interest of the female apparatus, are interesting from a taxonomic point of view and have been suggested as a systematic characteristic (Snodgrass 1941, Peck 1937, and Crampton 1919). In addition, the internal organs merit study, particularly as regards the female, in which conditions necessary for production of mature ova are so often dependent on the presence of the host as a pre-ovipositional food source. In the present account, the internal and external structures are described in some detail, and a brief account of an important physiological factor governing the development of the gonads of the female is given. (A) External Genitalia. (i) The Female (Figs. 17–20). The eighth and ninth may be considered as the genital segments, since it is the tergites of these which serve to support and protect the genitalia. The last abdominal spiracle is present in the eighth segment. Although in both of these segments the tergites are not markedly modified, the sternites are greatly altered to form the

ovipositor lancets and sheaths (Fig. 18). The ninth segment is normally telescoped into the eighth, and bears the anus at its dorsal extremity. The tenth and eleventh segments are represented by two atrophied plates basal to the anal protruberance, and just above these plates are the paired anal cerci (a.c.). Pivoted at the base of the ninth segment is the broadened, platelike base of the first valvifer (v.f.1.). This structure articulates with the second valvifer (v.f.2) behind, and at the base of the flattened, plate-like portion, it tapers off to the thin, flexible outer portion forming the first valvula (v.1) of the ovipositor proper. The internal portion of the first valvula arises as an internal prolongation of the base of the first valvifer. These two thin rods fuse to form one of the paired first valvulae or ovipositor lancets. The second valvifer is partly covered by the posterior margins of the ninth abdominal segment, and articulates near its base with the first valvifer. Below this, it expands to form a large, incurved plate which serves as a point of attachment for the muscles operating the genitalia during oviposition. Posteriorly, this plate bends back to form the second valvulae which fuse posteriorly to form the third section of the ovipositor proper. At its dorsal extremity, about midway between the anus and the ventral extremity of the ninth abdominal segment, the second valvifer gives rise to two lateral setose sheaths (the third valvulae) which serve to cover, and protect the ovipositor lancets (valvulae). The lancets are barbed at their tips and are secured together by paired hooks fitting into sockets (Figs. 19–20). (ii) The Male (Figs. 21–22). In the male, the ninth abdominal segment has both tergal and sternal elements, but has no pleurae. As in the female, the tergite has no spiracles. Behind the ninth abdominal segment is the basal ring (br), a collar-like sclerite encircling the bases of the parameres (pm). In Diadromus it is not complete dorsally, and is carinate on its ventrodistal edge. The parameres are very large, broad, setose plates, with their ventro-distal edges reflexed, almost completely covering the inner genitalia. The reflected edges, together with the slightly incurved, pointed tips of the parameres, probably serve a clasping function during copulation. The copulatory ossicles consist of a large basal volsella (vo), and a small apical segment, the sagitta (sa). The volsella itself is prolonged into an incurved horn distally, and this opposes the incurved and slightly larger sagitta. The sagitta is armed with short, stout spines, while the tip of the volsella bears long bristles, and a few placoid sensillae. Between and dorsal to the volsellae is the more or less cylindrical aedagus (ae), consisting of a pair of penis valves. In Diadromus, the parapenes (pa) which usually appear as supporting structures to the aedagus proper in Hymenoptera, are not differentiated from the penis valves. Distally, the margins of the penis valves are reflexed and hooked ventrally, to form a trumpet-shaped and downward-projecting opening.

(B) Internal Genitalia. (i) The Female (Figs. 23–25). The ovaries are paired, each consisting of four ovarioles (ov). The ovarioles run back and upwards, surrounding the gut and meeting dorsally to terminate in a common terminal filament (tf) towards the middle of the abdomen. Each ovariole is made up of an inner epithelial layer, which serves to transmit nutriment in solution to the inside of the ovariole for the nourishment of the developing ova, and an outer fibrous layer. The ovariole in Diadromus is of the polytrophic type. At its distal extremity is the germarium, consisting of oogonia and undifferentiated cells which later form the nutritive groups and follicular epithelium. The oocytes become surrounded by follicular epithelial cells, and each such follicle alternates with a group of nutritive cells (nc). Each follicle is broken at its upper end by a small opening (micropile, m) through which a cytoplasmic stalk (ns) passes to the nutritive cells above. By the time the ovum is completely developed, the nutritive cells have been exhausted of all cytoplasmic content, and the follicular epithelium has given place to a thin, flexible chorion. Below, the ovarioles end in short stalks, which unite to form paired oviducts (od) which lead by means of a broad, short vagina (vg) to the ovipositor. About half-way between the point of junction of the paired oviducts and the base of the ovipositor, the spermatheca (spt), a thick-walled, almost spherical sac, opens through a short duct into the vagina. At its distal extremity the spermatheca receives a short duct from the biramous spermathecal gland (spgl.). Accessory glands are absent. (ii) The Male (Figs. 26–30). The testes are paired, ovoid bodies lying above the gut in front of the rectum. They are united anteriorly, and terminate in a common filament. Each testis consists of an outer fibrous layer (fc) in which is embedded the much-coiled vas efferens (ve), and an inner, spongy mass in which are embedded the sperm cysts. Each testis apparently consists of only one large sperm-tube. The stages in the development of the sperms are readily traced. Below the apical cell (ape), the spermatogonia (spg) are seen in clusters in the spongy medulla. These groups are followed by follicles containing spermatocytes (spc), below which in order are the spermatids (sptd), and sperms (spm). No follicular epithelium has been observed surrounding these groups, although such epithelium may be present. The mature sperms lie in their follicles with their heads towards the opening of the vas efferens. The sperm has a head about one-third of its total length, and slightly bulbous at its middle. The convoluted vasa efferentia appear to be without definite cell walls and definite nuclei are not apparent. The vasa defferentia (vd), however, have a definite epethial lining composed of highly granular cells with large nuclei, and are surrounded by a heavy layer of circular muscle fibres. The paired vasa defferentia run down and back, one on each side of the hind-gut, to enter paired vesiculae seminales (vs) below the rectum. Each vesicula is pyriform and

trabeculate in section. They are lined with densely granular cells with very small nuclei. Below, they taper to ducts which almost immediately unite to form a common ejaculatory duct leading direct to the aedagus. The ejaculatory duct is lined with columnar epithelium and is surrounded with a Thick muscular layer of circular fibres with a few longitudinal fibres. No accessory glands are present, but throughout the length of the ejaculatory duct runs a glandular ridge (glr) which probably serves the same function. A Factor Governing the Development of the Female Gonads. During the commencement of the study of the internal genitalia of the female, difficulty was experienced in obtaining well-developed ovaries. Females were mated, then fed on honey and pollen solution (made from “bee-bread”) for several days before dissection. Invariably under these conditions the ovaries were found to be poorly developed, with no mature ova present. On dissecting females which had been allowed to oviposit in host pupae for several days, it was found that ovaries were well developed, and contained as many as eight mature ova at one time. It was known that females were in the habit of feeding on host pupae before active oviposition, and it appears that this habit is necessary for the maturation of the ova. Apparently the essentials of diet are obtained only from the fluid contents of the host pupa, and no foods tried could replace this natural source of nutriment. Such phenomena are not uncommon among female insects, and are probably the rule rather than the exception. Flanders (1935) mentions similar habits as regards a member of the Pteromalidae, and also quotes various other authors on the subject, while Lloyd (1940) mentions the same factor with Diadromus. The influence of mating on maturation is of no importance apart from the influence on the sex ratio of the progeny. Ova develop parthenogenetically, but all the progeny are males. Bibliography. Berlese, A., 1909. Gli insetti, vol. 1. Societa Editrice Libraria, Milan. Bischoff, H., 1927. Biologie der Hymenopteren. Verlag von Julius Springer, Berlin. Crampton, G. C., 1919. The Genitalia and Terminal Abdominal Structures of Males and the Terminal Abdominal Structures of the Larvae of “Chalastogastrous” Hymenoptera. Proo. Ent. Soc. Wash., vol. xxi, no. 6, pp. 129–151. Flanders, S. E., 1935. An Apparent Correlation Between the Feeding Habits of Certain Pteromalids and the Condition of Their Ovarian Follicles. Ann. Ent. Soc. Amer., vol. xxviii, pt. 4, pp. 438–444. Lloyd D. C., 1940. Host Selection by Hymenopterous Parasites of the Moth Plutella maculipennis Curtis. Proc. Roy. Soc. Lond., Ser. B, no. 853, vol. cxxviii, pp. 451–484. Peck, O., 1937. The Male Genitalia of the Order Hymenoptera (Insecta), Especially the Family Ichneumonidae. Canad. Journ. Res., Sectn. D, vol. xv, pp. 221–274. Pratt, H. D., 1935. Studies on the Ichneumonidae of New England (Hymenoptera). Part I. Journ. N.Y. Ent. Soc., vol. xxxviii, no. 2, pp. 155–194. Snodgrass R. E. 1935. Principles of Insect Morphology. McGraw-Hill Book Co., New York. —– 1941. The Male Genitalia of Insects. Smithsonian Misc. Coll., vol. 99, no. 14. Wigglesworth, V. B., 1939. The Principles of Insect Physiology. Methven and Co., Ltd., London.

Index to Lettering of Plates. A.—anal vein. ac.—anal cercus. ae.—aedagus. anep.—anepisternum. apc.—apical cell. apm.—anepimeron. ar.—areola. ax.—axillary sclerite. bas.—basareola. bc.—basicostal sclerite. bcl.—basicubital sclerite. bls.—blood sinus. bp.—basal pad of interstitial cells. br.—basal ring. C.—costa. cd.—cardo. cln.—colon. clp.—clypeus. cm.—circular muscle. cr.—crop. cu.—cuticle. Cu.—Cubitus. ex.—coxa. ed.—ejaculatory duct. ep.—epithelium. ephy.—epipharynx. eps.—episternum. fc.—fibrous layer. fd.—fold in wing. fe.—follicular epithelium. ga.—galea. gl.—glossa. glr.—glandular ridge. hg.—hind-gut. hm.—hammulus. il.—ileum. ktm.—katepimeron. kts.—katepisternum. lat.—lateral area. lb. plp.—labial palp. lc.—lacinia. lm.—labrum. lms.—longitudinal muscle. ltc.—lateral cervical sclerite. M.—media. m.—micropile. md.—mandible. mes.—mid-gut. meso.—mesosternum. mlp.—malpighian tubule. mp.—middle pleural area. mpl.—median plate. mpm.—mesepimeron. mps.—mesepisternum. mtps.—metapostscutellum. mts.—metascutellum. mtsc.—metascutum. mx. plp.—maxillary palp. nc.—nutritive cells. ns.—nutritive stalk. od.—oviduct. oc.—oesophagus. oeinv.—oesophageal invagination. ooc.—oocyte. ov.—ovariole. ovm.—ovum. pa.—parapenis. pet.—petiolarea. pgl.—paraglossa. pgr.—peritrophic groove. ph.—pharynx. plg.—palpiger. pls.—pleural suture. pm.—paramere. pmb.—peritrophic membrane. pr.—pronotum. pres.—prescutum. prmt.—prementum. prp.—parapsidea. prps.—parapsidal suture. prpt.—prepectua. prs.—prosternum. pscl.—parascutellum. ptc.—prepectal carina. pvent.—proventriculus. R.—radius. rect.—rectum. rectg.—rectal gland. rect. v.—rectal valve. rgr.—regenerative cells. sa.—sagitta. sc.—scutum. scl.—scutum. scv.—scutellar fovea. sp.—spiracle. spa.—spiracular area. spc.—spermatocytes. spg.—spermatogonia. spgl.—spermathecal gland. spm.—sperms. spt.—spermatheca. sptd.—spermatids. smt.—submentum. st.—stipes. tf.—terminal filament. tg.—tegula. tr.—trabeculae. trm.—tranverse muscle. ts.—testis. v.—valvula. vd.—vas deferens. ve.—vas efferens. vf.—valvifer. vg.—vagina. vm.—vertical muscle. vo.—volsella. vs.—vesicula semiualis.

Diadromus (Thyraeellia) collaris Grav. Fig. 1.—Thorax. Lateral. Figs. 3, 4 and 5.—Wing-coupling apparatus. Figs. 6 and 7.—Pteralia of fore- and hind-wings. Fig. 2.—Thorax. (a) Ventral, (b) Dorsal.

Diadromus (Thyraeela) collaris Grav. Fig. 8.—Mouth-parts. Fig. 9.—Entire insect. Longitudinal vertical section.

Diadromus (Thyraeela) collaris Grav. Fig. 10.—Phaiynx. Transverse section. Fig. 11.—Oesophagus. Transverse section. Fig. 12.—Mesenteron. Transverse section. Fig. 13.—Ileum. Transverse section. Fig. 14.—Rectum. Transverse section. Fig. 15.—Proventriculus and oesophygeal invagination. Longitudinal vertical section. Fig. 10.—Rectal vahe. Longitudinal vertical section.

Diadiomus (Thyraeclla) collaris Grav. Fig. 17.—Terminal segments and external female genitalia. Lateral. Fig. 18.—External genitalia. Female. Fig. 19.—Tip of ovipositor. Fig. 20.—Coupling hooks of valvula. Fig. 21.—Terminal segments and external male genitalia. Lateral. Fig. 22.—External genitalin. Dorsal at right, ventral at left.

Diadromus (Thyracella) collaris Grav. Fig. 23.—Internal genitalia. Female. Fig. 24.—Longitudinal section of ovariole. Fig. 23.—Photomicrograph of ovum and nutritive cells in ovariole. Fig. 26.—Internal genitalia. Male. Fig. 27.—Longitudinal section of testis. Fig. 28.—Longitudinal secion of vesicula seminalis. Fig. 29.—Transverse section of Vas defereus. Fig. 30.—Transverse sectionof ejaculatory duct.