Insect external structure
Insect internal structure
Development of insects
Relationships of insects
Insect identification > Trichoptera
The caddice (sometimes spelled caddis) flies, as the members of this order are usually called, are rather soft-bodied insects ranging in size from less than an eighth of an inch to an inch or more in length.
The wings, though much reduced in a few cases, are almost always large and well developed, with numerous longitudinal veins, but few cross veins.
They are membranous, the front pair somewhat leathery, and all are more or less densely covered with hairs which in some species are rather scale-like in form. The hind wings are usually broader than the front pair and when not in use are sometimes folded lengthwise. The position of all the wings when at rest is with their hinder margins together over the back of the insect and their costas down at the sides of the body, upper faces sloping downward and outward like a house roof.
The mouth-parts of the adult are poorly developed, though evidently modified from the chewing type, and it is probable that little if any food is taken in this stage. The antennae are generally well developed, and in some species they may be several times as long as the body. The legs are quite long and slender.
The larvae somewhat resemble small caterpillars in form. They are nearly all found in water, chiefly that of ponds or slow-running streams, though a few inhabit rapid currents. The abdomen is soft, the chitinous skin being delicate, and the larvae therefore construct cases of various materials as a protection for this portion of the body.
The Trichoptera may be defined as Insects which as adults have rather soft bodies; four membranous wings with numerous longitudinal and few cross veins, and more or less closely covered by hairs, folded over the body like a house roof when at rest. Mouthparts rather rudimentary. Antennae and legs quite long, the former sometimes exceptionally so. Larvae living in eases, nearly always in the water. Metamorphosis complete.
The adult caddice fly, though having well-developed wings, is not a strong flier and these insects are therefore most frequently found near water.
The eggs are, at least usually, laid in clusters in a mass of jelly and are probably dropped into the water. On hatching, the larvae begin the construction of cases in which to live. The materials of which these are made differ according to the species of caddice fly concerned and vary greatly. Some take pieces of leaves which have fallen into the water; others select veins of the leaves and similar sized straws and put them together crisscross, something like the logs of a log house; some species use the finest sand for this purpose, others coarse gravel, and still others use a mixture of long and short pieces of plants so that the ends of the longer ones extend some distance behind the end of the case.
The case itself is usually straight but in some species it may be curled and resembles a small snail shell. Indeed this resemblance is so close that, in one instance at least, such a case was actually described as that of a shell! The materials, whatever they may be, are held together by silk spun by the larva, coming from silk glands within the body and poured out through an opening close to the mouth. Within the case the larva lives, crawling about by extending its head and thorax out of the front end so that its feet can be used, and dragging the case along.
Some caddice fly larvae make simpler houses than these. Such species live in rapid water and there fasten a few tiny stones under rocks by their silk and between these spin a silken tube in which to live.
Close to this they spin more or less funnel-shaped webs, the mouth upstream and so arranged that tiny animals swept down by the current within the outer limits of the funnel come within reach of the larva lying in its tube. While the food of these larvae is carnivorous, in most of the species plant materials are consumed.
The larvae, in most cases, breathe by tracheal gills which are slender filaments, frequently grouped in clusters and attached to the abdominal segments. Other structures present in some species are also suspected of being concerned with respiration.
When full-grown, the caddice worm forms a sort of lid or door grating across the front opening of its case, though not complete enough to prevent water from entering and supplying the insect with the oxygen it needs. After pupation in its case the adult swims to the surface and grasps some object, from which it takes its flight. In some species it is apparently the pupa which, when ready to become the adult, comes to the surface and passes its final molt there.
The Trichoptera is quite a large group of insects and representatives of it are found in almost all parts of the world. Probably not many more than a thousand species have been described, as they do not appear to be of any economic importance unless their consumption of decaying vegetable matter in pools can be considered as desirable, but it is very likely that there are from five to ten thousand kinds in existence. Their cases have been found as fossils and adults have also been preserved in this way.
The Trichoptera are evidently closely related to the Lepidoptera in many ways and are undoubtedly, with the last named order, divergent descendants from common ancestors. Some Lepidoptera so closely resemble Trichoptera, in fact, that they have been placed in the latter group.
They also have many resemblances to the Neuroptera, but their connection with this order is plainly more remote, and sufficient time has elapsed since the divergence of the present Neuroptera and Trichoptera from their common ancestors to permit the development of many differences.