NATURAL HISTORY OF BHOMPSTON POND. 221 



burrows in the mud lined with a silky secretion and open at 

 each end. Having left their burrows perhaps through 

 insufficient aeration, these larvae are frequently seen near 

 the surface of the pond, swimming by very forcibly bending 

 and straightening themselves alternately. Close together 

 beneath the head there are two hooked appendages, and at 

 the extremity of the body two hooked foot-like organs used 

 in grasping the inside of the burrow. On the penultimate 

 segment are four blood gills, and four shorter and thicker 

 ones in the last segment of the body. Another very interest- 

 ing dipterous larva was found in the most stagnant parts 

 of the pond, though not very frequently. It is the larva of 

 Eristalis tenax, a most clumsy grey-coloured creature with 

 seven pairs of very short retractile hooked legs used in burrow- 

 ing in the mud, in which the animal lives. The posterior 

 end of the body is prolonged into a slender retractile tube 

 containing a structure that telescopes into it. Two tracheal 

 tubes pass through the entire length of this tail, by means of 

 which the larva is able to keep into communication with the 

 air while burrowing in the mud at variable depths. When 

 the tail is retracted the tracheal tubes can be seen coiled in 

 the posterior part of the body. This larva lives on the 

 decomposing matter of the sediment. 



Perhaps the most beautiful larva is that of Culex (fig. 72). 

 The insect, at this stage of its existence, suspends itself beneath 

 the surface of the water, though when disturbed it quickly 

 sinks, but returns to the surface by a series of jerks of the 

 abdomen ; when stranded it wriggles away sideways in a 

 very curious manner. It is an astonishing sight to see the 

 movements of the mouth appendages as they sweep food 

 (vegetable organisms, &c.) into the mouth. The long, narrow 

 abdomen, consisting of ten segments bearing bundles of elegant 

 hairs, terminates in four plates (a), and with many long seise. 

 The eighth segment bears the respiratory tube (b), at the 

 end of which the tracheal system opens to the air by two 

 stigmata. In the pupa (fig. 73) the abdomen ends in two 

 swimming organs, and curves beneath the large thorax, the 



