THE EXTERNAL STRUCTURE OF THE LEPIDOPTEROUS LARVA. 31 



mental membranes allow the segments to move freely upon each 

 other. The body segments are usually sub-divided into subsidiary 

 rings or sub-segments, which also move more or less freely upon each 

 other. The sub-segments are divided again into still smaller solid 

 portions, which have a certain amount of freedom, and are technically 

 called sclerites. 



A general description of a typical caterpillar or larva now becomes 

 necessary. We find that the caterpillars of Lepidoptera are usually 

 long and cylindrical, being, however, somewhat flattened on the 

 ventral surface. They may be considered as being composed of a head, 

 thorax and abdomen, as in the imago or perfect insect, although the 

 distinction between thorax and abdomen is not, as previously noted, 

 distinctly marked. The cylindrical shape of the larva depends upon 

 the fact that the larval skin contains fluid under considerable pressure. 



The head is a somewhat horny, compact, oval case, and is furnished 

 with a number of appendages about the oral opening. It is made up 

 of four (or more) segments, which, however, are not distinguishable 

 after hatching. On each side of the head are, usually, six simple ocelli, 

 arranged in lunular form on the cheeks. Tile mouth consists of a 

 labrum, mandibles, maxillae (with maxillary palpi) and labium (with 

 labial palpi). 



The thorax is composed of three segments (those following the 

 head), which are known as the pro-thorax, meso-thorax, and meta- 

 thorax (or post-thorax) respectively. In all larvae which burrow under- 

 ground or feed internally, and in many others, which do not, the dorsum 

 of the pro-thorax is protected with a hard, corneous plate, often, indeed, 

 extending to the meso-thorax and meta-thorax. Each of the three 

 thoracic segments bears on the ventral surface a pair of more or less 

 horny legs (the true legs), which have five joints and terminate in a 

 single claw. 



The abdomen consists of the last ten segments of the caterpillar's 

 body. They are very similar, in general appearance, to the thoracic 

 segments, but the tubercles, or little chitinous hair-bearing knobs 

 which they carry, are usually somewhat differently arranged, and they 

 never give rise to true legs. Some of the abdominal segments, however, 

 bear on their ventral surface a pair of stout fleshy protuberances, called 

 pro-legs or claspers ; these prolegs are really extensions of the integu- 

 ment, and have, on their free surface, a number of hooks. The number 

 and development of the prolegs, and the arrangement of their terminal 

 hooks vary considerably, different patterns distinguishing the several 

 families, and even genera. Very little use is made of the true legs 

 for purposes of progression, this being accomplished almost entirely 

 by means of the prolegs. The terminal segment of the abdomen is 

 known as the anal segment. On either side of the first thoracic, 

 and of the first eight abdominal segments, is a tiny opening called a 

 spiracle. The spiracles are round, oval, or longitudinal in shape, and 

 are the channels through which respiration is carried on. Eeguiarly 

 placed on certain parts of the body are to be found little chitinous, 

 hair-bearing knobs (sometimes modified into fleshy elevations), which 

 we have already said are called tubercles. These may give rise to 

 single hairs, but sometimes to exceedingly close and dense fascicles. 

 These tubercles are often strikingly modified at each successive ecdysis 

 or change of skin. 



