240 THE MICROSCOPE AND ITS REVELATIONS. 



dust, soot, etc., which would otherwise enter the air-passages, are filtered 

 out; and this sieve may be formed by the interlacement of the branches 

 of minute arborescent growths from the border of the spiracle, as in the 

 common Fly (Fig. 433), or in the Dytiscus; or it may be a membrane 

 perforated with minute holes, and supported upon a framework of bars 

 that is prolonged in like manner from the thickened margin of the aper- 

 ture (Fig. 434), as in the larva of the Melolontha (Cockchafer). Not 

 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 spira- 

 cle 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. 



636. 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 

 FIG- 434. 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 resem- 

 bling the 'fin-feet' of Branchiopods ( 

 603), 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 with- 



spiracie of Larva of Cockchafer. in 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 direction; and the air taken-up by its tracheae- 

 is carried, through the system of the air-tubes of which they form-part, 

 into the remotest organs. This apparatus is a peculiarly interesting 

 object for the Microscope, on account of the extraordinary copiousness 

 of the distribution of the tracheae in the intestinal folds. 



637. The main trunks of the tracheal system, with their principal 

 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 

 ( 180), 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 vis- 

 cera; 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 



