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backward entire; then follow the legs, which are withdrawn from the old 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 
ee 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. 
Methods. 
Changes 
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. 
