MICROSCOPE INDOORS. 65 



coiled up when the insect is at rest, and is the 

 organ by means of which the creature sucks up its 

 nutriment from the flower. It has a series of lines 

 running across it. 



If the head of the common blow-fly be now 

 examined, it will be seen that the tongue, instead 

 of being elongated as in the latter instances, is 

 expanded laterally (Fig. 196, PI. 7). This is a very 

 beautiful object, and when viewed by transmitted 

 light, a series of spiral bands are observed to wind 

 across each half of the tongue. 



The head of the spider presents an interesting 

 development of the mandibles. These organs are 

 in pairs, each mandible consists of two joints : one 

 is small, sharp, and hooked ; whilst the other is 

 large and short, and contains within it a bag, or 

 poison-gland (Fig. 197, PL 7), so that when the 

 creature seizes its prey, the bag is pressed on, and 

 a drop of the poison exudes. This structure is 

 similar to what is met with in the poisonous ser- 

 pents, where a poison bag is seated at the base of a 

 tubular tooth. 



The head of the spider affords also a good 

 example of what are called simple eyes. Besides 

 the compound ones before mentioned, insects have 

 also these simple eyes (Fig. 201, PI. 7). They con- 

 sist of a single lens (a), and are placed in various 

 positions in the heads of spiders. 



The skin of the common garden spider is covered 

 with hairs. These appear to surmount a series of 

 concentric plates (Fig. 202, PI. 7). They vary in 

 form in different species of spider ; and the skin of 

 all should be examined for the purpose of observing 

 these differences. The web of the spider should also 

 be examined. The cords of these beautiful structures 

 which run from the centre to the circumference of 

 the web, are plain (Fig. 207, PL 7) ; whilst those 

 F 



