92 Bulletin of the Brooklyn Entomological Society Vol. XV 



in dragonfly larvae (suborder Anisoptera). Damselfly larvae, 

 of the suborder Zygoptera, breathe by means of caudal leaf-like 

 gills. 



In gross structure, the morphology of the rectal gills has been 

 likened to a Japanese lantern, that is, the gill lamellae are ar- 

 ranged in six rows. These six rows are possessed by all Ani- 

 sopteran larvae, but modified in several ways. Thus there may 

 be six rows of simple lamellae, or six rows of double lamellae. 

 There is another classification into undulate, implicable, foliate 

 and other types of gill lamellae, depending on the species. For 

 further details I must refer to the papers of Tillyard listed in the 

 bibliography. 



Histologically, the structure of one of the gill lamellae is as fol- 

 lows : The lamellae are supplied with tracheal trunks, from which 

 short branches supplied with many tracheolar loops extend into 

 the lamellae, i.e., afferent and efferent capillaries, similar to the 

 blood capillaries of the gills in fish. In cross section each plate 

 is seen to be a lamella, each side one layer in thickness, with a 

 very thin outer cuticle. The cell layer is a modification of the 

 rectal epithelium and forms a syncitium in which the tracheoles 

 are imbedded. The tracheal capillaries lead back to the collect- 

 ing tracheae, these to the tracheal trunks which carry the oxygen 

 to all parts of the body, there to be taken up by the smaller ves- 

 sels and to the tissue cells. The number of tracheolar loops in 

 such a rectal gill has been estimated as varying from 10,000 to 

 90,000, according to the type of gill possessed by the respective 

 species. 



As the gas is used up in the tracheae there is a diffusion from 

 the region of higher partial pressure outside (in the water) to 

 the region of lower partial pressure within, the gas going out of 

 solution at once in the lumina of the tracheoles. The motion of 

 the rectal gills is to supply a fresh current of water. The com- 

 position of the air in the tracheae is the same as in' the water, that 

 is, in the proportions of 65 parts nitrogen to 35 oxygen, as com- 

 pared to the atmospheric 80 parts nitrogpn to 20 oxygen. (This 

 is at zero temperature. The amount of oxygen in solution varies 

 with the temperature of the water, being greatest at zero.) The 

 latter composition has been determined with an apparatus de- 



