APHIDAE 



589 



organ may in some way replace the missing Malpighian tubes. 

 Another highly peculiar structure is the siphons, frequently 

 called nectaries, honey-tubes, or siphuncles. They are situated 

 on the dorsal aspect of the fifth abdominal segment, but exist 

 only in certain of the sub-families ; they are of very different 

 lengths according to the species, and are capable of movement ; 

 they open directly into the body cavity, though exceptional 

 openings into the body cavity are extremely rare in Insects. 

 They excrete a waxy matter, which first appears as oil- like 

 globules. It was formerly supposed that they were the means 

 of secreting the sugary matter, called honey-dew, so much prized 

 by ants and some other Insects ; but this is now ascertained 

 to be erroneous. This matter comes from the alimentary canal, 

 and is secreted in large quantities by some species, Blisgen having 

 observed that forty-eight drops, each about 1 mm. in diameter, 

 were emitted by a single individual in twenty-four hours.^ Certain 

 gall-dwelling Aphidae — Pemphigus, Chermes (Fig. 285), SeJiizo- 

 neura — possess numerous wax glands ; these seem to replace the 

 siphons, and excrete the peculiar, whitish flocculent matter that 

 is so conspicuous in some of these Aphids. 



Earlier anatomists failed to find any dorsal vessel, and it is 

 consequently reported in books to be absent. It has been, how- 

 ever, recently detected by Witlaczil, and Mordwilko states that 

 it does not differ from that of other Insects. 



We have already alluded to the fact that the mode of repro- 

 duction of Aphids leads to an unrivalled increase. This, however, 

 is not due to the prolificness of the individual, which, in point of 

 fact, appears to be considerably below the average in Insects, but 

 rather to the rapidity with which the 3'oung begin to reproduce. 

 This has been discussed by Huxley, Buckton, and others. The 

 first-named naturalist calculated that the produce of a single 

 AjjJiis would, in the course of ten generations, supposing all the 

 individuals to survive, " contain more ponderable substance than 

 five hundred millions of stout men ; that is, more than the whole 

 population of China." " It has since been contended that 

 Professor Huxley's calculation was much below the mark. 

 Although it is somewhat difticult to make a calculation dealing 

 adequately with the actual facts, yet it is clear that the increase 



iZ. CeniralM. xi. 1891, p. 193. 

 ^ See, inter alia, "Webster, / A^ew York ent. Soc. i. 1893, p. 119. 



