DERMAL GLANDS 



365 



"A muscle extends from a horseshoe-shaped place (a valve ?) in the middle of 

 the flat terminal plate of the honey tube, through this and down through the 

 abdomen to the ventral surface. By this muscle the honey tube is at times 

 erected, and we then find, as also when we lightly press the body of the insect, 

 lumps of crystallized sugar which have been expressed through the tips of the 

 honey tubes." (Zool. Anzeiger, 1882, p. 241.) 



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 reach- 

 ing 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 ob- 

 servers. Thus, as early as 1815, Kyber 

 stated that the Aphides expelled an excre- 

 mentitious substance through the "sap 



FIG. 3oti. Wax-secreting 

 larva of a saw-fly. 



FIG. 357. Lachnus frfrobi, and its two "honey" warts. 

 Gissler del. 



tubes." Burmeister states that the tubes give out a fluid which "dries gum- 

 like, but, so far as I have observed, has no peculiar taste." K&uunur, 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 Linne", 

 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 larvae 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 modi- 

 fied or specialized to form secretory unicellular glands. Such are 



