MICROSCOPE IN-DOORS. 63 



Let us now seek another too common inhabitant 

 of London houses, the bed-bug, and, having de- 

 capitated him, submit his head to a low power. 

 He, too, is a biting creature ; and you will observe 

 that his jaws are finer than those of the flea, and 

 are like a pair of excessively fine sharp hairs 

 (Fig. 190, PI. 7); they are inclosed in a sheath, 

 from whence they are projected when used. In 

 the same sheath is the tongue, which performs the 

 double office of sucking up the blood of its victim 

 and depositing in the wound an acrid and irri- 

 tating secretion. The antenna and eyes of the bug 

 are also worthy of examination. From the latter 

 will be found projecting minute hairs. 



A still more despised animal may now be sought. 

 It also belongs to a large family, and 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. Disgust- 

 ing as connected with want of cleanliness, they 

 are, nevertheless, perfectly harmless. The head and 

 mouth (Fig. 191, PI. 7) 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 may be now 

 examined for the sake of comparison (Fig. 192, 

 PL 7). 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, arid capable of receiving impression from 

 external objects. The little lenses terminate on 

 the convex surface of the eye, presenting an im- 

 mense number of hexagonal facets (Fig. 203, PL 7). 

 In the common house-fly, there are said to be 



