122 ENTOMOLOGY 



in essentially a spiral manner, though the continuity of the so-called 

 spiral thread is frequently interrupted. These elastic threads, or 

 t&nidia, serve to keep the trachea open without affecting its flexibility. 



On the outer surface of the epithelium is a thin structureless basement 

 membrane. 



Tracheoles. The ultimate divisions of the air- tubes (Fig. 178) are 

 extremely delicate tubes, or tracheoles, which rarely end blindly, but 

 anastomose with one another, forming a capillary network of confluent 

 tubes, measuring less than o.ooi mm. in diameter, and filled, not with 

 air, but with a fluid. Respiration takes place doubtless by means of 

 the tracheoles rather than the tracheae. 



In a caterpillar, these capillary tubes spread out over the surface of 

 the cells of the silk-glands, according to Wistinghausen; and penetrate 

 into the gland-cells themselves, according to Holmgren; other authors 

 differ also as to the relation of the ultimate branches of the air-tubes to 

 the cells which they serve. 



The tracheoles consist of (i) a well developed peritracheal membrane, 

 which spreads out web-like between the bases of the tubes; and (2) a 

 chitinous intima without taenidia; the tracheoles being connected with 

 the tracheae proper by means of (3) transition cells. 



Unlike tracheae, the tracheolar tubes do not arise directly by in- 

 vagination, but develop each within a single cell of the epithelium of 

 a trachea. 



Respiration. The external signs of respiration are the regular open- 

 ing and closing movements of some of the spiracles and the rhythmic 

 contraction and expansion of the abdomen. During contraction, the 

 dorsal and ventral walls approach each other (Fig. 179) and during 

 expansion they separate. The tergum moves more than the sternum 

 in Coleoptera and Heteroptera, and vice versa in Locustidae, Odonata, 

 Diptera and aculeate Hymenoptera. The width of the abdomen usu- 

 ally changes but little during respiration, for the tergal and sternal 

 movements are taken up by the pleural membranes which, as in the 

 grasshopper, infold at contraction and straighten out at expansion. 

 Other respiratory movements occur, but they are of minor importance. 



The rate of respiration increases or diminishes with the activity of 

 the insect and with temperature and other conditions. In six specimens of 

 Melanoplus differentiate , held between the fingers, the thoracic spiracles 

 opened and closed respectively 34, 43, 45, 54, 60 and 61 times per 

 minute. Four individuals of M. femur-rubrum under the same circum- 

 stances gave 70, 78, 90 and 92. 



