INSECTA. 387 



the knee-joints ; in the bug it is tlic product of a single pyriform 

 gland, situated in the centre of the metathorax, and opening between 

 the hind-legs. Other insects have anal glands, in the usual form of 

 tubes, which, in the Gyr'midce, e. g., emit a very fetid fluid. In the 

 mole-cricket the anal glands consist of small lobular bodies, opening 

 into a reservoir for their secretion. In the Carahid<e and Staphy- 

 li/iidce the anal glands are ramified, or are vesicular, and have long 

 excretory ducts opening into muscular reservoirs, which expel, or, in 

 tlie Brachinus, explode the excretion, which, in the latter Bombar- 

 dier-beetles, immediately becomes gaseous on its ejection.* 



The wax of bees is formed in little scales between the hind-pair of 

 legs. The saccharine secretion of the Aphides, much sought after by 

 certain ants, exudes from teat-like processes near the vent. The 

 pungent caustic acid (formic) of the true ants, is elaborated by a 

 glandular apparatus in the anal region. The termites have not this 

 excretion, and the great ant-eater refused to feed on the British 

 species of Formicidce presented to it during its captivity at the 

 London Zoological Gardens, probably on account of their peculiar 

 acid. The phos[)horescent organs of the glow-worms {Lampyridte) 

 and fire-flies (^Eluteridce) consist of a mass of spherical cells, filled 

 with a finely granular substance, and surrounded by numerous 

 tracheal ramifications. This substance, which by day-light fills, in 

 tlie glow-worms, a portion of the abdominal cavity, shines through 

 the thin integument of the ventral arcs of the last abdominal 

 segments. In the fire-flies, the luminous substance shines through 

 two transparent spots on the dorsal arc of the prothorax. The light 

 brightens and dims synchronously with the acts of inspiration and 

 expiration ; it appears to be due to a slow species of combustion kept 

 up by the oxygen of the surrounding tracheaj.f 



LECTURE XVIIL 



GENERATION OF INSECTS. 



In the generative organs of insects, as in those of plants. Nature 

 seems to have been prodigal in her power of producing endless 

 varieties of forms out of one common type of organ ; these varieties 

 being subservient all the while to one common end or office. The ana- 

 logy of the reproductive parts of the insect to the reproductive flower 

 is the more striking, from the brilliant colours which the essential 



'o' 



* CCM. t XXIV. p. C32. 



c c 2 



