II 



FLIES WITH AQUATIC LARV^l 



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 are 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 are 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. 



