120 



AN INTRODUCTION TO ENTOMOLOGY 



.^C^ 



1 



These are hair-like or more or less plate-like expansions of the body- 

 wall, abundantly supplied with tracheas and tracheoles. Figures 136 

 and 137 represent a part of a tuft of hair-like tracheal gills of a 

 larva of Corydalus and figure 13 S a plate-like tracheal gill of 

 a naiad of a damsel-fly. In these tracheal gills the tracheoles 

 are separated from the air in the water only by the dehcate 



wall of the tracheal 

 gill which admits of 

 the transfer of gases 

 between the air in the 

 tracheoles and the air 

 in the water. 



Tracheal gills are 

 usually borne by the 

 abdomen, sometimes 

 by the thorax, and in 

 case of one genus of 

 stone-flies by the head. 

 They pertain almost 

 exclusively to the immature stages of insects ; but stone- 

 flies of the genus Pteronarcys retain them throughout their 

 existence. In the naiads of the Odonata the rectum is 

 supplied with many tracheae and functions as a tracheal gill. 



2. Respiration of Parasites 



It is believed that internal parasitic larvae derive their air from air 

 that is contained in the blood of their hosts, and that this is done by 

 osmosis through the cuticula of the larva, the skin of the larva being 

 furnished with a network of fine tracheae (Seurat '99). 



Fig. 138. — Tracheal gill of a damsel- 

 fly: A, entire gill showing the 

 tracheas; B, part of gill more 

 magnified showing both tracheae (T) 

 and tracheoles (t). 



3. The blood-gills 



Certain aquatic larvag possess thin transparent extensions of the 

 body wall, which are filled with blood, and serve as respiratory organs. 

 These are termed blood-gills. 



Blood-gills have been observed in comparatively few insects; 

 among them are certain trichopterous larvce; the larva of an exotic 

 beetle, Pelobius; and a few aquatic dipterous lar\^as, Chironomus and 

 Simulium. It is probable that the ventral sacs of the Thysanvira, 

 described in the account of that order, are also blood-gills. 



