EXTERNAL STRUCTURE OF THE BUTTERFLY LARVA. 19 



the most modified are the 8th and 10th ; in the Satyrids the latter is 

 modified to form a pair of pointed projections extending out behind 

 the larva. The 3rd, 4th, 5th, 6th and 10th abdominal segments 

 each bear a pair of prolegs, the last pair often known as the anal 

 claspers. 



The spiracles are tiny holes for the admission of air into the 

 tracheae or breathing-tubes ; they are placed in pairs (one on each 

 side) of the prothorax and the first eight abdominal segments. Each 

 spiracle leads, by means of a little tube, into a larger longitudinal one so 

 that all the tracheae connected with the spiracles are brought into connec- 

 tion ; injury to one spiracle, therefore, throws the work on others and 

 does not kill the larva. It will be observed that the spiracles on the 1st 

 thoracic and 8th abdominal segments are invariably larger than the 

 others, which are usually equal among themselves. The reason for 

 this is obvious, for each has to supply air to a much larger area, the 

 head and thoracic segments being dependent on the prothoracic, and 

 the 8th, 9th and 10th abdominal segments on the 8th abdominal, 

 spiracle. -It is to be observed also that these have usually a different 

 position from the others, being generally placed much higher than the 

 other spiracles. 



The caterpillar has, as we have noted, three pairs of true legs, one 

 pair situated on each of the thoracic segments. These are usually 

 small, jointed, horny, and provided with terminal hooks, but often so ill- 

 developed as to be comparatively useless for walking purposes. For this 

 purpose, and to support the long cylindrical abdomen, the skin on the 

 underside of the 3rd, 4th, 5th, 6th, and 10th (anal) segments of the 

 abdomen is prolonged downwards to form the prolegs and anal claspers 

 (or false legs), and, to make them more effective for walking purposes, they 

 are composed of joints which are partly retractile one into the other ; 

 the end joint of these prolegs is provided with minute hooks, the 

 arrangement of which is most important as giving clues to the line of 

 evolution of the various groups of butterflies. At the same time they 

 enable the larva to cling tenaciously to its foodplant. In the most 

 highly developed butterfly larvae these hooks exist only along the inner 

 margin of what is now a flange, but what was once a circular pad, and 

 the larvae of the skippers still have an almost complete circle of hooks 

 on the prolegs, hence the conclusion that the butterflies have been 

 derived from lepidopterous ancestors of a low or generalised type such 

 as the Hepialids, etc., which possess somewhat similar prolegs, and not 

 from the higher or more specialised moths which have the same type 

 as the other butterflies, i.e., excluding the skippers. 



We have already referred to the primary tubercles and the hairs 

 carried by them in the newly-hatched larva. It may be well now to 

 note them a little more particularly. The simplest form consists of a 

 little chitinous knob bearing a single hair or seta. Sometimes these 

 are modified so that a slightly raised base bears several hairs, in addition 

 to the primary seta, when we get a tubercular wart ; again they may be 

 modified into spines, fascicles of hair, etc. Examination of the dorsum 

 (or back) of a larva will show, at least on the abdominal segments 1-8, 

 two rows of chitinous-based hairs, or spines, running down the length 

 of the body, two on either side of each segment, those in front, rather 

 nearer the middle line of the back than those behind ; the front ones 

 are known as tubercles i, or the anterior trapezoidals, the hinder ones 



