118 ILLINOIS BIOLOGICAL MONOGRAPHS [436 



Only one species, Oryssus occidentalis Cresson, has been recognized in 

 the immature stages. The definitions given here are based on the descrip- 

 tions and figures of this species published by Rohwer and Cushman (1917). 

 It is quite possible that future studies may prove them inadequate for 

 the identification of the larvae of other genera and species yet to be dis- 

 covered. 



Oryssus Lateeille 



Larvae small; epicranial suture faint, arms obsolete; clypeus cres- 

 centic, narrow; fronto-clypeal and clypeo-labral sutures distinct; labrum 

 more than twice as wide as long, with shallow mesal emargination; an- 

 tennaria distinctly elevated, antacoria extensive, mound-like; antennae 

 small, mamma-like; mandibles strongly chitinized, curved, narrow mesal 

 dentis larger than lateral dentes, these subequal in size, sharp; maxillae 

 fleshy, sub triangular, unsegmented lobes; annulation on dorsum indistinct, 

 with apparently two annulets, venter with one; sublateral lobes distinct, 

 extending the entire length of the segments; spiracles visible from dorsal 

 aspect; segments transversely raised and with a few minute tubercles. 



Orysius occidentalis Cresson. — Color white with mandibles and chit- 

 inized ridges near the mouth brown; head one-third as wide as the widest 

 segment of the body — the fourth abdominal segment; maxillae with minute 

 brownish spots bearing about three sensory papillae; labium with about 

 four stout setae on each side of meson; prothorax declivous toward the 

 head, forming straight line with the latter in profile, on dorsum subequal 

 in length to mesothorax; metathorax half as long as mesothorax; lengths 

 of abdominal segments as follows: 8, (1, 9), (2, 7), (4, 6), 5, 10 on dorsum, 

 (6, 7, 8), 5, 1, 3, (2, 4), 9, 10 on venter, 8, 7, 4, (1, 2, 5, 6), 3, 9, 10 on latus; 

 tenth abdominal segment one-fourth as wide and one-third as high as the 

 fourth and sixth segments respectively; dorso-cephalic margin of mesothor- 

 acic and eighth abdominal segments distinctly, and of metathoracic and 

 abdominal segments 5, 6, 7, 9, 10 slightly convex, and of abdominal 

 segments 2 and 3 concave; ventro-cephalic margin of abdominal segments 

 1, 6, 7, 8 distinctly, and of 2 and 9 slightly, concave, of 3, 4, and 5 convex, 

 of 1 and 6 with a distinct convex emargination on each side of meson; 

 abdominal segments with the distance from spiracles to dorsal surface 

 uniform on lateral aspect and much shorter than the distance from spiracles 

 to the ventral surface, the latter variable and increasing caudad to the sixth 

 segment and diminishing thereafter; "each thoracic and abdominal segment 

 has dorsally at each side of the middle a low, transverse elevation sur- 

 mounted by a transverse row of four or five short, stout back-pointing 

 spines"; setiferous elevations on abdominal segments 1-7 and 9 and on 

 metathorax near the caudal margin of the segments, those of prothoracic 



