MICROSCOPE IX-DOORS. 85 



each mammal and bird seems to be attended with 

 its peculiar louse. Two species are found in dirty 

 and diseased conditions of the human body. Dis- 

 gusting as connected with want of cleanliness, they 

 are, nevertheless, perfectly harmless. The head and 

 mouth, drawn in figure 198, indicate that these 

 creatures are adapted to live on the secretions of 

 the skin. The above animals all belong to the 

 much larger group of creatures adapted to live as 

 parasites upon other animals. 



The head of the common gnat, figured at 199, 

 in plate 7, may be now examined for the sake of 

 comparison. In this creature, the eye of the insect 

 may be studied. It is what is called a compound 

 eye, and is composed of innumerable small lenses ; 

 each one of which is connected with a twig of the 

 optic nerve, and capable of receiving impressions 

 from external objects. The little lenses terminate 

 on the convex surface of the eye, presenting an 

 immense number of hexagonal facets. These are 

 seen at figure 210, plate 7. In the common 

 house-fly, there are said to be 4,000 of these facets; 

 and in the cabbage-butterfly 17,000. The antennae 

 of the gnat are very beautiful ; and, in fact, these 

 organs in insects afford an endless variety of forms. 

 At their base, in the gnat, is seen a round process 

 on which these are seated, and it has been supposed 

 that they are organs of hearing. Whether they 

 are organs of hearing or not, it is very certain that 

 they are organs of touch, and the creature is very 

 susceptible of the slightest stimulus applied to 

 them. 



The head of the honey-bee may be now examined ; 

 and if a careful dissection is made of its mouth, a 

 marvellous apparatus is unfolded to view, which is 

 exhibited in figure 201, plate 7, At the base is 

 seated the so-called mentum, and on each side are 



