MICROSCOPE IN-DOORS. 87 



The head of the common garden spider (Eperia 

 diadema) 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 ; so that when the creature seizes its prey, 

 the bag is pressed on, and a drop of the poison 

 exudes. This organ is represented in figure 204, 

 plate 7. This structure is similar to what is met 

 with in the poisonous serpents, where a poison-bag 

 is seated at the base of a tubular tooth. 



The description above given is the generally 

 received one ; but Mr. John Blackwall, our greatest 

 authority on spiders, considers the use of the term 

 " mandibles " to parts entirely without the mouth 

 objectionable ; he has accordingly bestowed the 

 name of "falces" upon them. Some carefully- 

 conducted and interesting experiments of his on 

 their so-called poisonous secretion seem to throw 

 great doubts on the propriety of regarding them in 

 this light, and he has been led to consider that the 

 purposes of it may rather be to deaden pain and 

 still the struggles of a captured animal, as chloro- 

 form is given previous to and during operations on 

 human beings. 



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 drawn at figure 208, 

 plate 7. They consist of a single lens, as seen at 

 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, seen at figure 209, plate 7 

 They vary in form in different species of spider ; 



