DIPTERA. 



273 



organs the Hippoboscidse, according to Verrill, are 

 able to run forward, backward, or sideways. 



The forest-fly or horse-tick, Hippobosca equina 

 (Fig. 223), belongs to this family. Its mode of 

 development differs from that of all other insects, 

 and remotely imitates some features in the embryonic 

 development of mammals. The oviduct has a sac- 

 like enlargement, \vithin which the larva is developed 

 and nourished by a milk-like secretion. The larval 

 and incipient pupal states are passed within the body 



Fig. 223. 



of the parent, so that when the young insect is born it 

 is covered with a puparium. 



The young of even the generalized forms of Diptera 

 are as a whole farther removed from the Thysanuri- 

 form type than those of any other group. The sec- 

 ondary larval form, which in the case of the Diptera 

 is always footless and often an almost headless maggot, 

 has complete possession of the younger stages. As 

 Friederich Brauer has pointed out, the general absence 

 in the larvae of Diptera of the thoracic legs, even 



