PHYLUM ARTHROPODA. SPIDERS AND THEIR ALLIES 



269 



the pedipalps and walking legs. The first pair 

 of appendages are called chelicerae (Figs. 

 162 and 163). In many species, they are 

 composed of two parts: a basal segment and 



a terminal claw or fang. Poison glands are 

 situated in the chelicerae of the tarantulas, 

 but in most spiders they are in the cephalo- 

 thorax. The poison (venom) they secrete 



Cephaiothorax- 



nr 



Abdomen - 



Intestine 



Poison gland 



Sucking stomach 

 Stomach 



Heart 



"Liver' 



Ostium 



Stercoral 

 pocket 



Malpighian 

 tubule 



Spinneret 



Chelicera 



Mouth 



Spiracle 

 rachea 

 Silk glands 



Seminal receptacle 



Figure 162. Internal structure of a spider as seen with the left side of the body removed. 

 (Modified from Lueckart.) 



passes through ducts that open on the fangs 

 of the chelicerae; the venom is used to kill 

 their prey and as a means of defense; it is 

 strong enough to kill small animals and, in 

 some species, to injure and kill larger ani- 

 mals. Although spiders in general have for 

 centuries been considered very poisonous, 

 the black widow is the only one in this coun- 

 try capable of causing death in man. The 

 second pair of leglike appendages are the 

 pedipalpi; their basal parts, called "maxil- 

 lae," are used as jaws to press or chew the 

 food. The pedipalpi of the mature male are 

 also used to transfer sperms to the female. 

 There are 4 pairs of walking legs (Fig. 

 164) . Each leg consists of 7 joints: ( 1 ) coxa, 

 (2) trochanter, (3) femur, (4) patella, (5) 

 tibia, (6) metatarsus, (7) tarsus. It is 



Figure 161. Facing page, some representatives of the 

 class Arachnoidea. The figures are not drawn to scale. 



terminated by two- or three-toothed claws 

 (Fig. 163), and often a pad of hairs, the 

 claw tuft, which enables the spider to run on 

 ceilings and walls. 



The sternum lies between the legs, and a 

 labium is situated between the "maxillae." 

 The eyes, usually 8, are on the front of the 

 head (Fig. 163). The mouth is a minute 

 opening between the bases of the pedipalpi; 

 it serves for ingestion of liquids only. 



The abdomen is connected with the 

 cephaiothorax by a slender waist ( peduncle ). 

 Near the anterior end of the abdomen, on 

 the ventral surface, is the genital opening, 

 which in some female spiders is covered by 

 a flat plate called the epigynum. On either 

 side of the epigynum is the slitlike open- 

 ing of the respiratory organs or book lungs. 

 Some spiders also possess tracheae which 

 open to the outside through spiracles near 

 the posterior end of the ventral surface (Fig. 



