SEXUAL CHARACTERISTICS OF THELIA aol 
thickness. This envelope constricts between the embryos and 
‘the chains are broken up into separate individuals, each of which 
develops into a larva surrounded by its nutritive envelope. The 
details of this growth and differentiation will be left to a subse- 
quent paper. 
If the Thelia nymph is stung during the first or second instar, 
the abdomen of the fourth instar will be filled with forty to 
sixty larvae bent in a half-circle, ventrad and measuring 0.75 
mm. from the cephalic to the caudal end. Each embryo is 
enclosed in a semitransparent envelope of cells which doubt- 
lessly serves as an organ of nutrition and respiration. The 
mouth parts are not chitinized at this stage. In the fifth instar 
or the adult Thelia the larvae reach their maximum development 
as internal parasites (fig. 1, p. 548). They grow tremendously 
and acquire large, hard, brown, chitinous mouth parts, well- 
defined stigmata, and a circumcephalic plate (Keilin and Thomp- 
son, 715) which in its dorsal region is brownish and thrown into 
many transverse folds. On account of the prominent mouth 
parts, we may designate the larvae of this stage as megagnathic 
larvae. Surrounded by fat, they are packed close to one another, 
about thirty-five being in contact with the abdominal sterna of 
the host, fifteen or more occupying the region lateral and dorsal 
to the digestive tube, while occasionally a few are present in the 
thorax. Reaching their maximum size, they distend the ab- 
dominal segments of the host to such an extent that the inter- 
segmental membranes are clearly visible between the abdominal 
sclerites. This must certainly interfere with the respiratory 
movements of the host. 
The megagnathic larvae have a well-defined alimentary tract 
which ends blindly caudad. The tract is filled with shining 
crystals which appear white in reflected light. These crystals 
are kept in constant motion by peristaltic waves which may be 
viewed nicely in living larvae in Ringer’s solution. A deep 
constriction of the digestive tube runs caudad, then reverses and 
runs cephalad. The crystals in the tube are very insoluble, 
persisting in specimens kept for several years in 80 per cent alco- 
hol and resisting in section-making all the ordinary reagents. 
