DEVELOPMENT OF PAEACOPIDOSOMOPSIS 35 



or among the tissues therein, a large number of groups of sex- 

 ual embryos. Each group results from the fact that the embryos 

 arising from a single component tend to stick together. The 

 groups vary in size and shape. Sometimes they form flat or 

 plate-like structures (figs. 78 to 80). More frequently they are 

 spherical in shape, which has given rise to the term 'ball' stage 

 in mj^ notes. The size of the group is determined })y the number 

 of embryos present, and these var}^ from two to as high as seventy. 

 In one lot of seventeen groups I counted the following num- 

 bers: 2 (two), 6, 10 (two), 12 (two), 15, 16, 18 (two) 20 (two), 

 22 (two), 25, 70. 



The embryos develop rapidly from the eighteenth day on, 

 and sometimes between twenty-second and twent3^-fourth days 

 reach the early larval stage. They then escape from their 

 capsules into the body cavity of the caterpillar. Once free, the 

 larvae proceed to devour the contents of the host, first eating 

 the fatty tissue, and finally devouring the various internal or- 

 gans. The last of these to disappear are the nervous system 

 and the intestine. In destroying the internal organs, the larvae 

 consume such portions as are dissolved by the action of their 

 salivary secretions. The undissolved parts consist largely of 

 the chitin of the tracheae. They also destroy all of the body 

 wall except the superficial layer of chitin. 



The larvae pupate on about the twenty-eighth dscy. During 

 pupation the non-digested content of the caterpillar hardens 

 and forms the thin-walled, oval chambers in which the parasi- 

 tic larvae lie and in which they undergo their transformation 

 into pupae. According to some observers, a thin cuticular layer 

 from the larvae forms an inner lining to the chamber and se ves 

 as a sort of puparium. The layer of chitin of the caterpillar 

 is perfectly transparent and at first is very flexible. Later, 

 as drying takes place, it shrinks in on the walls of the chambers 

 and becomes hard and rigid, the whole thus forming the typi- 

 cal mummified carcass, characteristic of polyembryonic para- 

 sites. Under laboratory conditions the parasites emerge from 

 the pupae on the forty-seventh day. 



