STREP SIPTERA 549 



over the plants on which these insects Hve till they find a nymph of 

 their host species. In the case of stylopids that infest social insects 

 the problem is obviously not so difficult, especially if the triungulinids 

 leave their host while it is in or near the nest. But those stylopids 

 that infest solitary nest-building species are beset with more serious 

 difficulties. It is believed that parasitized female bees and wasps 

 are so weakened that they do not build nests ; hence the triungulinids 

 issuing from them, and from males as well, must attach themselves 

 to other females of the same species in order to be carried to a nest 

 where they can find their appropriate victims. This transfer is 

 probably made in the flowers visited by these insects. 



When a triungulinid finds a larva or a nymph of its host species 

 it quickly bores into it, and begins its parasitic life. The most com- 

 plete account of the metamorphosis of a stylopid yet published is 

 that of Xenos vespdrum by Nassonow ('92). An abstract of this 

 author's results is given by Pierce ('09, pp. 47-48) ; the more important 

 features of them are the following. 



The campodeiform triungulinid grows rapidly after entering the 

 body of its host ; at the first molt it loses its legs and becomes scarabcci- 

 form ; later the body becomes cylindrical. From this point the develop- 

 ment of the two sexes is different. In the case of the females, there 

 are seven lar\^al instars ; in the fifth instar the head and thorax are 

 fused, forming a cephalothorax ; the seventh instar pushes its cephalo- 

 thorax out between two of the abdominal segments of the host ; the 

 skin of this instar becomes the "puparium," in which the adult female 

 is inclosed, and which she never leaves ; the adult female is larviform ; 

 there is no pupal stage in this sex. In the case of males, the headand 

 thorax of the fifth instar are fused, forming a cephalothorax; the 

 seventh instar is inclosed in the skin of the sixth, and has strongly de- 

 veloped appendages; for this reason it may be termed a prepupa; 

 during the seventh stadium the cephalothorax is exserted between 

 two abdominal segments of the host ; the true pupa is formed within 

 the skin of the seventh instar; the adult male thrusts off the cap of 

 the puparium and emerges as a winged individual. 



The manner in which the female is fertilized, inclosed as she is in 

 a puparium, has not been determined; it has been suggested that the 

 seminal fluid is discharged into the space between the venter of the 

 female and the pupariiun, the brood chamber. If this is true, the 

 mobile spermatozoa probably pass from the brood chamber through 

 the genital apertures into the abdominal cavity, where the eggs are 

 massed free. The slit in the cephalothorax of the puparium, through 

 which the triungulinids escape, may serve for the introduction of the 

 seminal fluid into the brood chamber. 



The order Strepsiptera is well represented in this country. Leng 

 ('20) lists ninety-seven American species , and doubtless there are many 

 undiscovered species here. The described American species represent 

 five families and eighteen genera. 



Students wishing to study the classification of these insects should 

 consult the very complete monographs of the order by W. Dwight 

 Pierce ('09, '11, and '18), and other papers listed in these works. 



