II 



FLIES WITH AQUATIC LARV/E 



167 



the head may not take any harm from rubbing 



against hard objects, it can be 



completely retracted into the 



body. 



The most notable features of 



the mouth-parts arc a pair of 



mandibles with long curved 



teeth. The top of the head is 



defended by a strong shield, 



from the under side of which, 



along the middle line, hangs 



downwards a vertical plate. This 



plate, as well as the rest of the 



shield, gives origin to muscles, 



and especially to the powerful 



muscles which move the jaws. 



There is a pair of small antennae, 



and two rudimentary eye-spots on each side of the 



head. Towards the tail are three pairs of tapering 



prominences ; the hindmost pair is very long, and 



forms the extremity of the body. These appendages 



are supplied with rela- 

 tively large air-tubes, 

 and are probably of use 

 in aerating the blood. 

 Other means of respira- 

 tion arc provided in the 

 form of spiracles. As 

 in a good many other 

 Insect-larvae, which in- 

 habit water or burrow in the earth, there is only one 



pair of spiracles, which are situated at the hinder end 



Fig. 52. — Head of larva of 

 Dicranota, ventral surface. 



Fig. 53. — Head of larva of Dicranota, 

 retracted into thorax. 



