TICKS. 211 



which have been said to "exhibit such remarkable varia- 

 tions in their typical structure that they have been regarded 

 by some authors as forming a distinct order. The head 

 is received into a cavity in front of the thorax; it is 

 divided transversely into two parts, the anterior or smaller of 

 which supports the mouth and two small tubercles, almost 

 imbedded at the lateral angles, being rudimental antennse. 

 The mouth is composed of two curved setse, inclosed in a 

 tubular canal, covered by two narrow elongated coriaceous 

 plates, regarded by Latreille as palpi. The ocelli are wanting. 

 The body is short, flat, and very coriaceous; the wings are 

 either large or entirely wanting ; the nerves of the anterior 

 margin are very strong, but they are effaced behind. In the 

 winged species a pair of balancers are -also present. The 

 legs are very strong, and terminated by robust curved claws, 

 which are toothed beneath. The abdomen is composed of 

 a continuous leathery-like membrane, capable of very great 

 distension, which peculiarity is owing to the remarkable cir- 

 cumstance that the young of these insects are singly nourished 

 within the body of the parent, where they not only acquire 

 their full size, but actually assume the pupa state, under 

 which form, like very large eggs, they are deposited by the 

 female. This egg-like cocoon is at first soft and white, nearly 

 as large as the abdomen of the parent fly; but by degrees it 

 hardens, becomes brown, of a rounded form, and often notched 

 at one end, which is covered by a shining kind of cap, which 

 is detached on the insect's assuming the perfect state. This 

 cocoon is moreover entirely destitute of annular incisions, in 

 which respect it differs from those of other Dipterous Insects. 

 It is composed of the uncast skin of the larva, beneath which 

 the insect becomes a real inactive pupa, with the limbs of 

 the perfect insect laid along the breast, as in other species 

 which undergo the strict coarctate species of transformation. 



