INSECTS. 261 



their life, and which beyond doubt are of service in diminishing 

 weight during flight. These dilatations are oval or pear-shaped, and 

 occasionally a tubular trachea proceeds anew from their further 

 side. In the Apiarice amongst the Hymenoptera, the two lateral 

 main trunks of the air-canals in the abdomen are in this way 

 converted into large reservoirs of air. 



The stigmata are present in different numbers in the hexapod 

 Insects, but it is rare to find more than nine pairs of them; in 

 Dytwcus amongst the Coleoptera and in Locusta amongst the 

 Ortkoptera there are ten pairs (BiTRMEiSTER Handbuch der Ento- 

 mologie I. p. 175). Also in Gryllotalpa I found ten pairs, three in 

 the thorax and seven in the abdomen. These air-slits are small, 

 generally oblong fissures (like button-holes), often surrounded by a 

 horny ring (peritrema) with a cavity behind them which again, by 

 a second fissure whose posterior half can be retracted by muscles, 

 leads to the air-canals. In other instances there is no peritrema, 

 but the stigma is formed by a fissure between two lips, whose 

 edges are beset with hairs. Sometimes there are in the cavity of 

 the stigma special moveable horny plates (epiglottides STRAUS), 

 which can close the entrance of the air-canal that proceeds from it. 

 By means of the oblique position of the lips, of which one often 

 projects over the other, by means of the narrow opening, and of 

 the hair or down on their edge, the entrance of dust or other 

 small bodies into the stigmata is prevented, whilst the air alone is 

 admitted as through a sieve. From every air-slit, or its cavity 

 (vestibule) there arises an air-canal (trachee d'origine STRAUS) 

 which divides into numerous branches (in Scolopendra) , or proceeds 

 transversely after having given off one or two lateral main-stems. 

 These main-stems running along the length of the body, (in most 

 Insects there is only one on each side,) receive all the canals that 

 spring from the air-slits or fissures, and connect them together. 

 ! They give off the numerous branches which spread through every 

 i part of the body. The distribution of the air-canals after the 

 I manner of vessels is interesting ; by such a disposition of the 

 respiratory organs in Insects, the atmospheric air has access in 

 equal degree to every part of their body *. But it is too much to 



" In nobis et consimilibus sanguinis massct pulmones petit ... in insectis non totn 

 fftnyninis moles in pulmones conflult, sed inversa via pulmones ipsi, vasorum ritu, in 



