36 PREDACIOUS AND PARASITIC 



The Tracheal System. 



The general plan of the apparatus fulfilling the important 

 functions of respiration has been exhibited at Fig. 2 of PI. XV. 

 (Vol. 4). 



Two large and exceedingly delicate tubes (there marked b^ b, 

 and in PI. III., Fig. i, marked h, g), containing in their walls the 

 usual spiral elastic thread, traverse the whole length of the larva, 

 one on either side. Towards the extremity of the head, at a 

 point well invaginated when the larva is in a state of rest, these 

 tubes protrude, carrying with them a conical extension of the epi- 

 dermis, and presenting in some aspects (and especially when the 

 insect is in the act of extending itself) the appearance of a pair of 

 palpi. They terminate each in a narrowed, rounded point, fur- 

 nished with chitinous slits for the admission of air, the opening or 

 closing of which seems determined by a circular membranous 

 surface situated obliquely at the base of the point. 



Towards the posterior extremity the main tracheae approach 

 one another, and as at the head, pierce the epidermis, but together, 

 and in the central line, frequently forming a double wart-like pro- 

 tuberance. Each tube here terminates with a circular or oblong 

 ring of chitin, within which a drum-like membrane is partially 

 surrounded by three chitinous ridges set in the ring radially. Each 

 of these ridges, being pierced by a slightly serrated slit, forms an 

 efficient valve for the admission or exclusion of air, regulated by 

 the tension of the circular membranes, and giving the larva com- 

 plete control over its respiration. The volume of air included 

 within the main trachea and its extensive branches is sufficient to 

 enable the functions of life to be sustained with but little incon- 

 venience, even when immersion in water precludes the possibility 

 of obtaining fresh supplies. 



If, instead, the larva be placed in a cell only partially filled 

 with water, it will soon raise either the posterior or anterior valves 

 to a level with the surface, and thus resembles, in its method of 

 breathing, the larvae of the genus Eristalis. 



A curious adaptation of the breathing process of the larva 

 may be observed if a young and transparent specimen be kept in 

 a shallow vulcanite cell, covered by a loose glass, for several days. 

 It will do perfecdy well without food, but should be exposed to the 



