RESPIRATORY APPARATUS 



997 



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. 



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 con- 

 vey 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 branqfiiopods, 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 containing 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 direc- 

 tion ; and the air taken up by its 

 tracheae is carried through the 

 system of air-tubes of which they 

 form part into the remotest organs. 

 This apparatus is a peculiarly in- 

 teresting object for the microscope 

 011 account of the extraordinarily 

 rich distribution of the tracheae in 

 the intestinal 'folds. FlG 744 ._Spiracle of larva of 



The main trunks of the tracheal cockchafer, 



system, with their principal ramifi- 

 cations, may generally be got out with little difficulty by laying 

 open the body of an insect or larva under water in a dissecting 

 trough, and removing the whole visceral mass, taking care to leave 

 as many as possible of the branches, which will be seen pro- 

 ceeding to this from the two great longitudinal tracheae, to whose 

 position these branches will serve as a guide. Mr. Quekett recom- 

 mended 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 allowed to dry upon the slide, and should then be treated 

 in the usual way ; but their natural appearance is best preserved 



