SPIRACLES. 573 



lontha (cockchafer). $"ot unfrequently, the centre of the aperture 

 is occupied by an impervious disk, from which radii proceed 

 to its margin, as is well seen in the spiracle of Tipula (crane-fly). 

 In those aquatic larvae which breathe air, we often find one of 

 the spiracles of the last segment of the abdomen prolonged into 

 a tube, the mouth of which remains at the surface, while the 

 body is immersed ; the larvae of the Gnat tribe may frequently be 

 observed in this position. 



393. There are many aquatic Larvae, however, which have an 

 entirely different provision for respiration ; being furnished with 

 external leaf-like or brush-like appendages, into which the tracheae 

 are prolonged, so that, by absorbing air from the water that 

 bathes them, they may convey this into the interior of the body. 

 We cannot have a better example of this than is afforded by the 

 larva of the common Ephemera (day-fly), the body of which is 

 furnished with a set of branchial appendages resembling the "fin- 

 feet" of Branchiopods ( 368), whilst the three-pronged tail also 

 is fringed with clusters of delicate hairs which appear to minister 

 to the same function. In the larva of the Libellula (dragon-fly), 

 the extension of the surface for aquatic respiration takes place 

 within the termination of the intestine; the lining membrane of 

 which is folded into an immense number of plaits, each contain- 

 ing a minutely ramified system of tracheae ; the water, slowly 

 drawn in through the anus for bathing this surface, is ejected 

 with such violence that the body is impelled in the opposite 

 direction ; and the air taken up by its tracheae is carried, through 

 the system of air-tubes of which they form a part, into the 

 remotest organs. This apparatus is a peculiarly interesting 

 object for the Microscope, on account of the extraordinary co- 

 piousness of the distribution of the tracheae in the intestinal 

 folds. 



394. The main trunks of the Tracheal system, with their prin- 

 cipal ramifications, may generally be got out with little difficulty, 

 by laying open the body of an insect or larva, under water, in a 

 dissecting-trough ( 104), and removing the whole visceral mass, 

 taking care to leave as many as possible of the branches which 

 will be seen proceeding to this from the two great longitudinal 

 tracheae, to whose position these branches will serve as a guide. 

 Mr. Quekett recommends the following as the most simple 

 method of obtaining a perfect system of tracheal tubes from a 

 larva : a small opening having been made in its body, this is to 

 be placed in strong acetic acid, which will soften or decompose 

 all the viscera ; and the tracheae may then be well washed with 

 the syringe, and removed from the body with the greatest facility, 

 by cutting away the connections of the main tubes with the 

 spiracles by means of fine-pointed scissors. In order to mount 

 them, they should be floated upon the slide, on which they should 

 then be laid out in the position best adapted for displaying them. 

 If they are to be mounted in Canada balsam, they should be 



