COMMUNICATION" AMONG INSECTS McINDOO 557 



which are common to mammals, and sebaceous or oil glands (s), 

 common to birds and mammajs. The sebaceous glands usually open 

 around the base of hairs (A) in depressions called hair follicles, 

 and not into the cavity or lumen of the hairs. On the palms of a 

 human being sweat glands number 1,100 to the square centimeter, 

 and their total number in man is said to be about two million. 



It is the primarj^ function of the sebaceous glands to Rubricate 

 the skin, hair, and feathers of animals; if they have a secondary 

 function it may be that of recognition. When fresh, the secretion of 

 these glands, called sebum, is an oily semifluid, but on the surface 

 of the skin it solidifies, forming a greasy coating. 



Primarily, the sweat glands constitute an automatic device for 

 keeping the body temperature constant, while their secondary func- 

 tion is probably recognition. The perspiration is a clear, colorless 

 fluid with a salty taste, and characterized by different odors from 

 different parts of the body. In composition it is 99 per cent water, 

 the remaining 1 per cent consisting of urea, ammonia, fatty sub- 

 stances, and certain other compounds. Its neutral fats are said to 

 come from the sebaceous glands, although the sweat glands them- 

 selves secrete other fatty substances of which the volatile fatty acids 

 emit the peculiar odor characteristic of sweat. 



We can now understand why the odors emitted from two animals 

 differ. We can also understand how dogs, by smell alone, even in 

 total darkness, can distinguish their masters and the members of their 

 respective households from strangers. Besides the glands already 

 mentioned, animals have certain other glands whose secretions may 

 also be odorous and perhaps sexually attractive; for instance, the 

 glands connected with the genitals. 



Sweat glands and sebaceous glands are present in tlie higher ani- 

 mals but not in insects; therefore insects, so far as we know, do not 

 sweat. They belong to the cold-blooded animals, whose temperature 

 most of the time corresponds more or less closely to the temperature 

 of the air in which they live, and therefore do not need sweat glands. 

 Many insects, however, possess glands which are widel}' distributed 

 over the entire surface of the body and closely resemble sebaceous 

 glands. Since these glands are not needed for purposes of lubrica- 

 tion the}^ must have some other function, and it seems very reason- 

 able to suppose that they are used for recognition. Those of the 

 cotton boll weevil (fig. 4, A, p) illustrate this point. 



Among specialized glands of this type are the adhesive-secreting 

 glands (fig. 4, B) in the feet of many insects, i:)articularly of all 

 those that can walk on perpendicular surfaces or even upside clown. 

 Although the primary function of these glands is certainly to enable 

 insects to walk on smooth, perpendicular surfaces, a secondary use 

 is probably that of recognition. In ants, termites, and all the other 



