Fig. 357.—Lachnus strobi, and its two “honey” warts.—Gissler del.
Busgen, after careful research, denies that this is a sugar, but claims as the result of chemical analysis, that it is more like wax. He observed that on reaching the air the drops issuing from the “nectary” or “honey” tube stiffened almost instantly into a wax-like mass, which was easily crushed between the teeth, and had no taste at all. No sugar-like substance or urea could be detected. He therefore concludes that the secretion in question should be regarded as a wax-like mass, which agrees well with Witlaczil’s anatomical observations, and confirms the statements of previous observers. Thus, as early as 1815, Kyber stated that the Aphides expelled an excrementitious substance through the “sap tubes.” Burmeister states that the tubes give out a fluid which “dries gumlike, but, so far as I have observed, has no peculiar taste.” Réaumur, and also Kaltenbach, state that the “honey” does not issue from the tubes, but from the anus. Lastly, Forel emphatically states that “the two dorsal tubes of Aphides do not secrete a sweet fluid, but a gluey wax, which is not sought by the ants. Moreover the shield-lice and many leaf-lice have no such tubes, but yet are often sought by ants. The drops of sugar which the ants lick up are rather the excrement of the insects in question.” Hence the opinion first stated by Linné, that a sweet fluid is secreted by Aphides, must be abandoned.
On the other hand, Busgen, after careful observations, finds that the use of the sticky, waxen secretion is in reality a protective one, as he observed that when a larval Chrysopa rudely attacks the Aphides, they smear its face with the sticky wax, causing at least a momentary interruption in its attacks. He also observed that Aphides when invaded by coccinellid larvæ set their tubes in motion and besmear their heads and front part of the body. He thus seems to establish the fact that these tubes secrete a protective, sticky fluid.
i. Dermal glands in general
We have seen that certain of the hypodermal cells may be modified or specialized to form secretory unicellular glands. Such are those (trichogens) which secrete chitinous setæ, hairs, and spines, certain setæ in some insects being hollow and containing a poison (p. 187); others secrete wax, certain ones in Aphids “honey-dew”; in some cases dermal glands may excrete protective, sticky, or otherwise offensive matters, or may be depuratory, or facilitate the process of moulting.
There are other minute, unicellular, or compound dermal glands whose function is unknown.
Dermal glands may be segmentally or serially arranged. Thus Verson has detected a series of one or two pairs of unicellular glands near the stigmata in each thoracic, and the first eight abdominal segments of the silkworm (B. mori). In the earliest stages of growth of the caterpillar they give out oxalate of lime, and in later stages uric acid. They thus appear to act interchangeably with the urinary tubes, as excretory organs. They do not, however, carry their products directly outwards, but leave them between the hypodermis and cuticula, in order to facilitate the sloughing off of the latter in the process of moulting.
LITERATURE ON THE SECRETORY GLANDS
a. General
Sirodot, S. Recherches sur les sécrétions chez les Insectes. (Ann. sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–328, 12 Pls.)