201
[216]
THE EXTERNAL ANATOMY OF THE SPIDER.
by
W. RAE SHERRIFFS.
PROFESSOR OF ZOOLOĠY, UNIVERSITY COLLEGE, SOUTHAMPTON, ENGLAND.
46
A first view shows that the spider has a body and a number of legs arising from that body. A closer examination demonstrates that the leglike structures are not all alike and that the body is in two parts separated by a slender waist " (pedicel). The front part of the body is the united head and thorax (cephalothorax) and from it arise the limbs. The second portion is the abdomen. Both cephalothorax and abdomen are not, as in Insects, composed of definite rings or segments. A still closer scrutiny--which, in the case of minute specimens at least, demands the use of a lens to aid natural vision-reveals yet smaller structures on the large main parts above mentioned.
CEPHALOTHORAX.
This fore piece of the body is usualy smaller than the hind one, the abdomen, and when looked at from above the dorsal part (carapace) is seen to have a smaller front area separated by a groove from the rest. This cervical groove comes behind the head area which bears the simple eyes, usually eight in number, arranged variously in different kinds of spiders. and according to their number, nature, relative size and position to each other, are of great use in classifying spiders correctly. The spider eye is a single eye with one lens like the ocellus in some insects but quite different from the relatively enormous compound eye made up of thousands of eye elements so characteristic of the insect. Sometimes a spider may have two kinds of eyes, pearly white and black, the pearly ones being nocturnal and the black diurnal, i.e. for use by day.
On the under (ventral) side are the mouth parts consisting of (1) one pair of "jaws" (chelicerae) for seizing and killing the prey-for all spiders are carnivorous. They are the modified second antennae of the crustacean and not Insect mandibles. The chelicerae are two-jointed hollow structures, the upper joint containing a poison sac while the end joint is a movable sharp- pointed fang which folds like a knife-blade against the handle and which can be pushed into the victim, most commonly an insect; (2) A pair of sen- sitive "feelers" (pedipalps) used like insect antennae yet definitely limblike in structure. In the female the pedipalps are limblike but in the male the last joint of the six present is strangely and often elaborately modified for the transference of sperms, as already noted in
already noted in my last paper.
*
The pedipalp's six joints are the coxa, trochanter, femur, patella, tibia and tarsus.
The coxa joins the limb to the body and the last end joint is the tarsus which in the male is changed to a sex organ. So constant is the type of the male palpal organ that it is characteristic of the species and therefore very valuable in classification.
* H.K.N. Vol. II, No. 4, PP. 254-257.
The Hong Kong Naturalist.