594 



ZOOLOGY 



SECT. 



FIG. 483. Thorax and anterior abdominal 

 segments of a larval Ephemerid with 

 tracheal gills. HF , hind wings ; t,-l, tr*, 

 ^/a, tracheal gills ; tl, tracheal longitu- 

 dinal trunks; VF, fore wings. (From 

 Lang's Coiitptii-atifc Anatomy.) 



They are always protected against the entry of foreign particles 



by some means either by being surrounded by special bundles 



of hairs, or by being closed in 

 by a special sieve-like membrane. 

 In all cases they are capable of 

 being closed by muscular action. 

 In some Insects, mainly those 

 adapted for active flight, such as 

 the Hymenoptera, the tracheal 

 system is dilated in certain parts 

 of the body to form comparatively 

 large air sacs or air reservoirs (tb.). 

 In the aquatic larva? of some 

 Insects there is a series of soft 

 external, simple or divided, pro- 

 cesses the trachea I gills (Figs. 

 483) attached to the abdominal 

 segments, and richly supplied 

 with tracheae, which have no com- 

 munication with the exterior. 



The blood-vascular system 

 is, in comparison with the other 

 systems of organs, not very 



highly developed, the need of an elaborate system of vessels 



being greatly diminished by the way in which all the tissues 



and organs are supplied with oxygen 



through the system of tracheae. The 



blood is colourless or faintly yellowish or 



greenish, and contains colourless cor- 

 puscles. A contractile dorsal vessel or 



heart (Fig. 484) extends through the ab- 

 domen, immediately below the terga. Its 



cavity is divided internally into a series 



usually of eight chambers by a system 



of valves. In its walls are a series of slits 



or ostia, by which a communication is 



effected between the internal cavity and 



a surrounding pcricardial sinus. In front 



the heart gives origin to a main vessel, or 



aorta (a), by means of which the blood is 



conveyed throughout the body to enter a 



system of sinuses in free communication 



with the general body cavity, from the 



various parts of which it finds its way 



back to the pericardial sinus. 



The nervous system ( Figs. 482 and 485) is on the same general 



plan as in the Crustacea. There is a double supra-cesophageal 



Fm. 484. Heart of Cockchafer 

 (Melolontha). n, aorta ; 

 m, m, alary muscles. (From 

 Gegenbauer.) 



