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 Apiarie 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 
Dytiscus amongst the Coleoptera and in Locusta amongst the 
Orthoptera there are ten pairs (BuRMEISTER 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 st?gmata 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 (trachée 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 
part of the body. The distribution of the air-canals after the 
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 
i“ In nobis et consimilibus sanguinis massa pulmones petit... in insectis non tota 
sunguinis moles in pulmones confluit, sed inversa via pulmones ipsi, vasorwm ritu, in 
