368 TEXT-BOOK OF ENTOMOLOGY 



DEFENSIVE OR KEPUGNATORIAL SCENT-GLANDS 



While these eversible glands are not found in marine or aquatic 

 arthropods such 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 being repellent, warning, or defensive structures, the 

 odors they secrete 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 centre of distribution. 



As is now well known, the very delicate, attenuated highly vola- 

 tile 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 Er- 

 langen, 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 2JG( ^ wm of a grain of mercap- 

 tan. 1 The smallest particle of matter that can be detected by the 

 eye is sodium, when observed by the spectroscope, and this particle 

 is 250 times coarser than the particle of mercaptan which can be 

 detected by the human nose. 



In those Arachnida which are provided with poison-glands, those scent-glands 

 are absent, but in certain Acarina and Linguattilidre, which have no poison- 

 glands, there are various oil-glands, stiginatic glands, as well as scent-glands, 



1 Mercaptan is a mercury, belonging to a class of compounds analogous to alcohol, 

 having an offensive garlic odor. Methyl mercaptan is a highly offensive aud volatile 

 liquid. 



