122 ILLINOIS BIOLOGICAL MONOGRAPHS (440 



conditions in insects in general and in the Tenthredinoidea in particular, 

 the probable ancestral type of larva may be characterized as follows: 

 body cylindrical; segmentation distinct; annulation indistinct, annulets 

 few in number; head exposed, subglobose, distinct from the trunk; thorax 

 and abdomen more or less similar in structure excepting the three pairs of 

 thoracic legs, which are well developed and consist of five segments, 

 tarsal claws distinct; abdomen with twelve segments including the telson; 

 larvapods present on abdominal segments 1-10; antennae long, composed 

 of several segments; ocellarae present, one on each side of head; mouth- 

 parts well developed, maxillary and labial palpi segmented; tenth abdom- 

 inal tergum without caudal protuberances or suranal process; eleventh 

 abdominal sternum with a pair of segmented subanal appendages; ten 

 pairs of functional spiracles present, including metaspiracles; larvae free 

 leaf-feeders. 



There are, as was pointed out by Comstock (1893), two kinds of char- 

 acters of phylogenetic importance. "First, characters indicating differ- 

 ence in kind of specialization; and second, characters indicating difference 

 in degree of specialization of the same kind. The former will indicate 

 dichotomous divisions of lines of descent; the latter merely indicate 

 degrees of divergence from a primitive type." 



In determining the probable genetic affinities of the families of the 

 Tenthredinoidea, the following structures have been taken into considera- 

 tion: thoracic legs, larvapods, subanal appendages, ocellarae, antennae, 

 mouth-parts, suranal process, and metathoracic spiracles. The list does 

 not by any means exhaust the structures which might be employed for 

 this purpose, but it is believed that the structures listed offer the most 

 reliable and essential basis for the determination of a phylogeny based 

 upon larval characters. The significant changes in these structures are: 

 addition or reduction of parts; difference in degrees of development of 

 existing parts; and modifications in length, size, shape, and degree of 

 chitinization of the parts. These modifications have been interpreted 

 according to the Comstockian principles quoted above. 



The thoracic legs are among the most persistent structures in the 

 adult and larval stages of insects in general, and their absence is unques- 

 tionably an indication of specialization by reduction. It is likewise 

 reasonable to assume that any modification of the typical, simple, cylin- 

 drical, five-segmented condition as regards the form or the number of 

 segments is a sign of specialization. The legs of the larvae of the Pam- 

 philiidae approximate most closely the primitive condition in the number, 

 shape, and structure of the segments. The tarsal claw is straight and 

 very slender. In the Xyelidae the legs assume a condition different from 

 that of the Pamphiliidae. The differentiation of segments in size and shape 

 has proceeded further and the tarsal claws have become distinctly claw- 



