Insect Camouflage

New Zealand Herald, Volume LXXIV, Issue 22819, 28 August 1937, Page 10 (Supplement)

Insect Camouflage

Use in Defence and Attack

Specially Written for The New Zealand Herald by R. A. FALLA, M.A.

ONE gives little thought to the matter of insect life in winter until it is brbught to mind by the appearance of a few house-flies, a bluebottle or two, or perhaps a stray white butterfly, abroad on a sunny day. Then it becomes apparent, on reflection, that there is some continuity linking , the swarming insect population of last summer with the swarms that it is reasonably safe to predict for next summer.COLD DOES NOT KILL Methods of passing the winter are many and varied. It is not that cold weather kills off most insects and leaves only a few survivors to carry on; on the contrary, it requires very low temperatures indeed to kill an insect at any stage of development. As in the case of the cold-blooded vertebrate, the effect of low temperatures is rather to suspend tho vital energy of protoplasm without destroying it. IN HIDING PLACES The greatest mortality among adult insects occurs long before the onset of cold weather, usually som© time in tho autumn when they have completed their reproductive functions. Some of them, however, probably those that matured late, will live on through the winter months, kept torpid in some hiding place by the lower temperatures, and it is these that bestir and show themselves on mild warm days, if any. But for the

most part the winter is passed by insects in tho more convenient stages of egg, larva or pupa. In these stages, too, the eircct of cold is only to retard development, and the various devices securing protection from cold as found in warm-blooded mammals and birds have no place in the economy of insects, nor of any other cold-blooded creature. TOUGH SKIN AND ODOURS

Tho protective devices which are so marked a feature of insect form are, then, largely of such a nature as to aid in escaping the attention of or actually deieriing, an enemy. Insects that.pass the winter in the pupa stage have cocoons that aro tough and resistant to attack. Where exceptions occur, as in the frail chrysalids of some butterflies, it is usually found that the pupa in question is protected by a deterrent odour. But mere toughness of pupa case or armour-like hardness of perfect insect are tho least subtle of the protective devices found in insects. Just as in human history metal armour tended to defeat its own end by impeding the wearer, so in the insect world it is found that heavily-mailed species are circumscribed in movement and distribution. RESEMBLANCE TO PLANTS A more successful means of protection would seem to exist in what is called "mimicry," a term which in a biological sense docs not apply to conscious attempts at imitation, but to marked natural resemblances between different animals, animals and plants, or animals and inanimate objects. Among insects ono of tho commonest forms is resemblance to plants or parts of plants, and of this the familiar stick insect is a striking example. Fairly common in New Zealand, these insects belong to the order Othoptera and are, there-

fore, related to grasshoppers, cockroaches and crickets. They differ very much in external appearance from all such insects, however, in having long, thin bodies, and generally no wings. Their resemblance to green and brown twigs is close, not only in the bark-like texture of the skin, but also in the way in which the legs stick out like branches. The whole resemblance is increased by their habit of feigning death when disturbed and they are often incredibly difficult to detect. "LIVING THORNS" The ordinary features of insect structure seem lost in the superficial likeness to the plant form and it is only by close scrutiny that head can be distinguished from thorax, and thorax from abdomen. The presence of sharp projections exactly like thorns gives to some species of stick insects _ further protection from possible enemies. Ihe disguise is complete and the stick insect relies 011 it by keeping still in the presence of danger and rarely seeks safety in retreat. The feet end in two sharp hooks between which are small cushions, an arrangement, enabling tho insect to retain its hold in spite of jerkings of the branch to which it clings. Tho New Zealand species, of which about 14 have been described, &re wingless, but in some of the larger tropical species wings, resembling leaves, are also found. The disguise seems to extend even to the eggs which are hard and seed-like. KALLIMA BUTTERFLIES Other common forms of protective colouration are found in the wing markings of moths, many of which when at

rest resemble bark so closely as to be indistinguishable against it. Others have the colour and pattern of mosses, others again of lichens which may in themselves be quite conspicuous but nono the less effective as a protection for the insect. Butterflies, usually bold and striking in the pattern of the upper wing surfaces, have frequently a subdued obliterative pattern on the undersides. In the normal position of rest, with wings closed over tho back, tho bright colours aro hidden and the more protective pattern is exposed. An extreme and striking example of this is provided by tho Kalliina butterflies of India and tho Malay Peninsula, in which the exposed undersides of the closed wings have not only the colour of a dead leaf, but its pattern complete with veins and midrib, and even assume a leaf shape complete with stalk. Not the least effective feature of such a colour scheme must be the bewildering transition seen by a pursuing bird when its colour-flashing prey seems there ono second and gone the next. PRAYING MANTIS In examples so far mentioned the advantage gained by obliterative colouring is purely defensive. It must be regarded as giving advantage also in aggression when found in such predacious insects as the praying mantis. Belonging also to the order Orthoptera .this familiar bright green insect when at rest has something of the colour and appeai'ance of a new leaf. Sitting motionless with the fore part of its body raised it awaits the approach of some other insect that it may seize with its powerful front legs. The waiting attitude has been likened to praying, hence the common name; in reality it is associated with preying. In New Zealand the praying mantis is an insect of the summer, its eggs hatching about November. The winter is passed in tho egg, dozens of which lie packed in little dry capsules that may be found attached to fences or the branches of trees from April onward.