66 INTERNAL ANATOMY OF INSECTS. 
In those of the Libellulince there are six. According to 
M. Cuvier, Reaumur, who mentions only Jour, overlooked 
the two lateral ones that are connected with the spira- 
cles a . The reason of this and other parts of their in- 
ternal structure I shall explain under the next head. In 
the grub of the gad-flies of the horse (Gastcrophili,) 
Mr. B. Clark discovered eight longitudinal trachea:, — six 
arranged in a circle and two minute ones, which appeared 
to him to terminate in a pair of external nipples (spiracles) 
in the neck of the animal b . This is a singular anomaly, 
as the other (Estrida have only a, pair of trachea: c . 
iii. Respiratory Sacs or Pouches. Besides their trachea 
and bronchia;, many insects are furnished with reservoirs 
for the air, under the form of sacs, pouches, or vesicles. 
These are commonly formed by the bronchial tubes 
being dilated at intervals, especially in the abdomen, into 
oblong inflated vesicles; from which other bronchial 
tubes diverge, and again at intervals expand into smaller 
vesicles, so as to exhibit no unapt resemblance — as Swam- 
merdam has observed with respect to those of the rhi- 
noceros-beetle — to a specimen oiFucus vesiculosus. Cuvier 
compares them in the Lamellicorn beetles in general to 
a tree very thickly laden with leaves d ; and Chabrier 
observes that they particularly occur in the intestinal 
canal e . This structure of the pulmonary organs may 
be seen also in the common hive-bee, and other Hymeno- 
a N. Diet. d'Hist. Nat. xvii. 541. Reaum. vi. 397. Plate XXIX. 
Fig. 8. shows three of them at a. 
b Essay on the Bots, $c. 23. t. If. 7, 32, &c. 
c Ibid. 49. Valisnieri i. 101. t. vi./. 4. &c. 
d Bibl. Nat. i. 149. a. t. xxix. /. a. Cuv. Anat. Comp. iv. 439. 
Malpigh. De Bombyc. t. iii./. 2. 
c Sur le Vol des Ins. c. ii. 33G. note 1. 
