770 



RESPIRATORY APPARATUS OF INSECTS. 



Spiracle of Larva of Cockchafer. 



it may be a membrane perforated with minute holes, and sup- 

 ported upon a framework of bars that is prolonged in like manner 

 from the thickened margin of the aperture (Fig. 349), as in the 



Larva of the Melolontha 

 Fig. 349. (Cockchafer). Not un- 



frequently, the centre of 

 the aperture is occupied 

 by an impervious disk, 

 from which radii pro- 

 ceeded to its margin, as 

 is well seen in the Spi- 

 racle of Tipula (Crane- 

 fly). — In those Aquatic 

 Larvae which breathe air, 

 we often find one of the 

 Spiracles of the last seg- 

 ment of the abdomen 

 prolonged into a tube, 

 the mouth of which re- 

 mains at ths surface 

 while the body is im- 

 mersed ; the Larvae of the Gnat tribe may frequently be observed 

 in this position. 



526. 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 (§ 497), 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 direction ; 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 interesting object for the Microscope, on account of 

 the extraordinary copiousness of the distribution of the Tracheae in 

 the Intestinal folds. 



527. 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 



