45] LEPIDOPTEROUS LARVAE — FR ACKER 45 



accordingly bear two or three setae (K, fig. 39) ; but neither is ever 

 multisetiferous. 



When the setae are increased in number new modifications are to 

 be noted in the cuticular areas to which they are attached. The most 

 common tendency is toward the development of tufts, in which case 

 each tuft is said to be borne on a verruca (L. verruca, ''wart") as 

 shown in Fig. 88. If the setae are so thickly grouped as to form a dense 

 upright bundle, the term verricule (Fig. 91), already in use in ento- 

 mology, describes their condition. In some cases the setae are increased 

 in number on a pinaculum without forming tufts, as on the prolegs of 

 notodontians and arctians (Fig. 100). As the term plate is already in 

 use for this structure and is not applied to other different conditions, 

 it is adopted here. 



More conspicuous than verrucae are the thorny processes bearing 

 spine-like setae, found in saturnian and nymphalid larvae (Fig. 74), 

 and for them the term scolus (Gr. k oAo?, "thorn") is particularly 

 appropriate. When the setae are borne on sharp lateral projections of 

 the scolus, these projections are known as spinules. 



As the word tubercle has been applied to each one of the above 

 structures by previous writers, it is deemed best to retain it as a general 

 term. 



Spiracles. — A classification of larvae based on the spiracles is still 

 unwritten. It is to be hoped that some future investigator will study 

 their structure and report the variations in different families. As yet 

 little use can be made of them altho differences in their shape, location, 

 and formation can not but be noted. One case in w^hich they are useful 

 is the pyrali-zygaenoid series in w^hich the spiracles become smaller and 

 smaller as the insects become more and more specialized. Certain of 

 these families, such as the Pyromorphidae, can not be easily distin- 

 guished from some Bombycoidea by the arrangement of their verrucae, 

 but their small, circular spiracles are very different. (Figs. 79, 104.) 



Prolegs. — Nearly two hundred years ago Reaumur figured four or 

 five forms of prolegs and the hooks attached to them. He made no use 

 of them in classification but the figures are more accuratel}^ drawn than 

 those of most later workers. Chapman was the first to call attention to 

 the fact that the crochets of Microlepidoptera are usually arranged in 

 a circle while those of the Macros are, in most cases, placed in a single 

 longitudinal row. 



Prolegs are normally present on segments 3, 4, 5, 6, and 10 of the 

 abdomen, one pair to each segment. In general statements only the first 

 four pairs are referred to. When it seems necessary to distinguish these 



