﻿110 Journal New York Ent. Soc. lvoi. hi. 



THE EVERSIBLE REPUGNATORIAL SCENT 

 GLANDS OF INSECTS. 



PLATE V. 



By a. S. Packard. 



While these eversible glands are not found in marine or aquatic 

 Arthropods as Crustacea or Merostomata (Limulus), they are often 

 present in the air-breathing forms, especially insects. In the winged 

 insects they are of frequent occurrence, existing under great variety of 

 form, varying greatly in position, and appearing usually to be in 

 immediate relation with their active volant habits. Their presence is 

 in direct adaptation to the needs and habits of their possessors, and be- 

 ing repellant, warning, or defensive structures, the odors they secret 

 being often exceedingly nauseous, they appear to have been called into 

 existence in direct response to their biological environment. The fact 

 that these singular organs do not exist in marine or aquatic Crustacea 

 suggests that the air-breathing, aerial or volant insects by these eversible 

 glands, usually in the form of simple evaginable hypodermic pouches, are 

 enabled to protect themselves by emitting an infinitesimal amount of an 

 offensively odorous fluid or ether-like spray which charges the air 

 throughout an extent of territory which may be practically illimitable 

 to the senses of their enemies. The principle is the same as in the 

 mephitic sulphuretted oil ejected by the skunks, the slight quantity these 

 creatures give out readily mixing with and charging the atmosphere 

 within a radius of many miles of what we may call the center of 

 distribution. 



As is now well known, the very delicate, attenuated highly volatile 

 odors exhaled are perceived by insects with extreme ease and rapidity, 

 the degree of sensitiveness to such scents being enormously greater than 

 in vertebrates, their organs of sense being developed in a corresponding 

 degree. Professors Fischer and Penzoldt, of Erlangen, have recently 

 established the fact that the sense of smell is by far the most delicate 

 of the senses. They find that the olfactory nerve is able to detect the 

 presence of ^,^60.000,000 of a grain of mercaptan.* The smallest particle of 

 matter that can be detected by the eye is sodium, when observed by 

 the spectroscope, and this particle is two hundred and fifty times 



* Mercaptan is a mercury, belonging to a class of compounds analogous to 

 alcohol, having an offensive garlic odor. Methyl mercaptan is a highly offensive 

 and volatile liquid. 



