THE ELYTRA 



125 



angle with the body and admit of the free play of the wings" 

 (Kirby and Spence). In the running beetles (Carabidae), also in the 

 weevils and in many Ptinidae, the hind wings are wanting, through 

 disuse, and often the elytra are firmly united, forming a single hard 

 shell or case. The firmness of the elytra is due both to the thick- 

 ness of the chitinous deposit and to the presence of minute chitinous 

 rods or pillars connecting the upper and lower chitinous surfaces. 



Hoffbauer finds that in the elytra of beetles of different families 

 the venation characteristic of the hind wings is wanting, the main 

 tracheae being irregular or arranged in closely parallel longitudinal 

 lines, and nerve-fibres pass along near them, sense-organs being also 

 present. The fat-bodies in the cavity of the elytra, which is lined 

 with a matrix layer, besides nerves, tracheae, and blood, contain se- 

 cretory vesicles filled with uric-acid concretions such as occur in the 



FIG. 139. Longitudinal section through the edge of the elytrum of Una cenea: gl, glands; 

 r, reservoir ; fb, fat-body ; m, matrix ; , upper, I, lower, lamella. After Hoffbauer. 



fat-body of Lampyris. There are also a great many glands varying 

 much in structure and position, such occurring also in the pronotum 

 (Fig. 139). 



Meinert considers the elytra of Coleoptera to be the homologues of the tegulse 

 of Lepidoptera and of Hymenoptera. He also calls attention to the alula 

 observed in Dyticus, situated at the base of the elytra, but which is totally 

 covered by the latter. The alulae of these beetles he regards as the homologues 

 of the anterior wings of Hymenoptera and Diptera. No details are given in 

 support of these views. (Ent. Tidskrift, i, 1880, p. 168.) 



Hoffbauer (1892) also has suggested that the elytra are not the homologues of 

 the fore wings of other insects, but of the tegulte. 



Kolbe describes the alula of Dyticus as a delicate, membranous lobe at the 

 base of the elytra, but not visible when they are closed : its fringed edge in 

 Dyticus is bordered by a thickening forming a tube which contains a fluid. 

 The alula is united with the inner basal portion and articulation of the wing- 

 cover, forming a continuation of them. Duf our considered that the humming 

 noise made by these beetles is produced by the alulets. 



Hoffbauer finds no structural resemblances in the alulae of Dyticus to the 

 elytra. He does not find "the least trace of veins." They are more like ap- 

 pendages of the elytra. Lacordaire considered that their function is to prevent 



