66 INTERNAL ANATOMY OF INSECTS. 



In those of the Libellulince there are six. According to 

 M. Cuvier, Reaumur, who mentions only^owr, 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 (Gasterophili,) 

 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 CEstrida 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 ofFucus 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- 



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. /. xxix. /. a. Cuv. Anat. Comp. iv. 439. 

 Malpigh. De Bombyc. t. iii./. 2. 



e Sur le Vol des Ins. c. ii. 336. note 1. 



