THE MECHANICAL CONCEPTION OP NATURE. 241 



seen in the darts made by liini in chase of the female. Food 

 probably is not necessary to these flies. They however consume 

 the pollen and honey of flowers. A rather complex but feebly con- 

 structed mouth leads to an attenuated stomach and small intestines. 



But as to the habits of the larva, all here is changed, and new 

 organs are developed suited to an aquatic life. 



"WTiere the egg of Eristalis tenax is laid is not yet certainly 

 known, but it must be deposited in pools of stagnant water con- 

 taining mud formed by the rotting of animal and vegetable matters 

 of a disgusting character. 



Baron Osten Sacken has shown that the distribution of this 

 insect is connected with the migrations of the human race ; and 

 this would seem to have some reference to the interest it shows in 

 human excreta, yet as the larv^ are found in tangled masses, in the 

 shallow parts of ponds visited by cattle, its food is not entirely 

 restricted to the former kind noted. The fat maggot-like larva 

 buries itself in the slime, in places where ordinary gills would fail 

 to allow of respiration. The singular modification of trachea? to 

 effect this pui'pose has been often shown, and the elastic character 

 of the long compound respiratory tube at the tail end has been 

 described by M. H. Viallaines and others. This long tube ends 

 in a single opening, fringed with bristles, which, through their 

 repellent action on the surface of the water, forms a kind of funnel 

 or depression through which, free from mud, the air passes into 

 the two somewhat capacious tracheal sacs in the semi-transparent 

 body of the grub. 



As the level of the water in these swamps rises and falls, a 

 beautiful provision is made to allow these tracheal tubes to stretch 

 some inches, or to contract, without kinking the double tubes 

 within, and clo.sing them. 



The larva of Eristalis is eyeless, and at the later stages of its 

 development it is capable of protruding seven pairs of serrated 

 false feet, which enable it to crawl to the shore and to bury itself 

 for pupation in the damp earth. 



The larva shows no conspicu.ous mouth-parts, but its anterior 

 end is furnished with three lobe-like lips (the labrum) plentifully 

 studded with recurved horny hooks, with which the animal rasps 

 up and reduces to a pulp the matters on which it feeds. 



The respiratory tube is often tied by the insect into curious 

 knots without interfering however with the internal calibration. 

 This knotting may be commonly seen when the larvae are com- 

 pelled to rise to the surface of deeper water Here they may hang 



