108 AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



early moulting gives no such advantage. 3. Young spiders moult rapidly 

 and easily, and with little loss of vitality ; therefore precautions against 

 dangers incident to that period are scant or wanting. 4. The method 

 of moulting is substantially the same in all species. 5. The 

 head and thorax moult first ; the old skin cleaves horizontally 

 about midway of the latter, the shells escaping backward and upward and 

 downward, the mouth parts adhering to the sternum moult ; the pedicle 

 splits longitudinally, holding the above parts severally to the upper and 

 lower front of the abdomen. The abdomen next follows, the shell escaping 

 backward entire ; then follow the legs, which are withdrawn from the old 

 sheaths downward by an interrupted series of muscular contractions and 

 strains, usually escaping entire and in succession, beginning with the first 

 pair. 



6. The number of moults varies according to species, from seven to 

 ten being the most common limit ; these are made at intervals more or less 

 regular, and at corresponding periods of time in individuals of 

 the same species or even genera. 7. The periodicity of moult- 

 ing is modified by amount of food, by temperature, and by causes 

 affecting the spider's normal health ; it is wholly suspended by the pres- 

 ence within the body of a larval parasite and by the prick of a wasp's 

 sting. 8. A period of relaxation and exhaustion, more extended and severe 

 as age increases, follows the moult. 9. Many species protect themselves 

 against this after-moult weakness by various precautions in accord with 

 their social nesting habits, such as creeping into crannies and leaf-rolled 

 dens* covering over burrows, et cetera. Sedentary spiders usually spin a 

 moulting frame. These precautions are commonly cotemporaneous with 

 the increasing difficulty of moulting. 



10. Changes in color and pattern, more or less decided, occur with the 

 successive moults. 11. The young males and females are scarcely distin- 

 guishable just after the first moult, but the difference grows more 

 distinct with successive moults until the last moult, when the 

 animals are mature and the sexual characters distinctly marked ; at this 

 time the male form most widely diverges from that of the typical young. 

 12. The changes of skin are often attended with loss of limbs or parts 

 thereof, and death sometimes results from inanition ; the attendant 

 weakness exposes the subjects to assaults of stronger congeners and alien 

 enemies, so that moulting thus becomes a factor of danger to individual 

 life, and so to the perpetuation of the species. 



IX. 



We pass now from the biological phenomena of the moulting period to 

 consider the physiological and histological processes and changes connected 

 therewith. The most complete study of these has been made by Mr. 



