232 



ANNELIDA AND ARTHROPODA: 



legged larva which later metamorphoses to the adult 

 form, or hatch directly to the adult form. 



CLASS ARACHNIDA (Arachnids) 



Diagnosis: eyes all simple; pedipalpi mostly sen- 

 sory, but male's often modified for sperm transfer; 

 without gills (Figure 14.9). 



The arachnids are mostly terrestrial, predaceous 

 animals with four pairs of legs plus head structures, 

 the pedipalpi, which often are sensory in nature. 

 The body is of two parts, except in one group, the 

 mites. The two body parts are the cephalothorax, a 

 uniting of head and thorax, and the abdomen. There 

 are no antennae ("feelers") or true jaws, and the eyes 

 are all simple. Respiration is by book lungs or 

 tracheae. Book lungs are so named because of their 

 construction, resembling leaves of a book. Tracheae 

 are air-tube systems whereby gases are conducted 

 directly between the atmosphere and the cells of the 

 animal. The sexes usually are separate, the males 

 often being smaller than the females. Most lay eggs; 

 there is no metamorphosis except for some slight 

 change in form in the Acarina. 



ORDER SCORPIONIDA (Scorpions) 



Diagnosis: body elongate; cephalothorax rela- 

 tively small, pedipalpi modified into pincers; cephalo- 

 thorax broadly fused to abdomen; abdomen elongate 

 and of 12 segments, the last six constricted (the tail) 

 and terminated by a poisonous stinger; 1 to a few 

 inches long (Figure 14.9). 



A scorpion's tail typically is curled over the back 

 and the stinger is thus directed forward in a position 

 of attack or defense. Pincers, modified pedipalpi, 

 usually are used to grasp anything that is to be 

 stung. Although two Arizona species are often con- 

 sidered more poisonous than a black widow spider, 

 most scorpions are not more dangerous than a wasp. 

 The popular belief that they commit suicide by sting- 

 ing themselves is as untrue as the belief that snakes 

 swallow their young to protect them. Scorpions pro- 

 duce living young, which stay a week or more upon 

 their mother's back. During this period, which is 

 ended by the first molt, the young do not feed. 



Scorpions inhabit warm, dry regions where they 

 are active mostly at night, preying upon insects, 

 spiders, and other scorpions. During the day they 

 hide in burrows or under objects on the ground. 



ORDER PSEUDOSCORPIONIDA (Pseudoscorpions) 



Diagnosis: scorpion-like, but with abdomen of 1 1 

 segments that are not modified into a whip-like tail, 

 abdomen not terminated by a stinger; minute, up to 

 3^4 inch long; produce poison (Figure 14.9). 



Although they bear no abdominal stinger, these 

 animals do possess poison glands in the pincers. The 

 poison probably is used to subdue the mites, psocids, 

 springtails, and other tiny animals they eat. They 

 share the ability of spiders to spin silk, spinning 

 nests during periods of helplessness, such as molting. 

 They live under objects on the ground; under tree 

 bark; in bee, termite, or ant nests; and in human 

 dwellings. Many are cave dwellers. Others can be 

 found under rocks along upper beaches, and some are 

 even intertidal. Pseudoscorpions are noted for their 

 habit of attaching to and being transported by flying 

 insects such as flies or beetles. 



ORDER PHALANGIDA (= OPILIONES) (Harvesfmen or 

 Daddy Longlegs) 



Diagnosis: somewhat spider-like; body oval, ceph- 

 alothorax broadly joined to abdomen; abdomen with 

 some indication of segmentation; pedipalpi not modi- 

 fied as pincers; nonpoisonous, but cephalothorax has 

 stink glands; body generally small, but legs up to 6 

 inches long (Figure 14.9). 



The daddy longlegs, or harvestmen, have been 

 given the latter name because of the time of year 

 when they congregate and are most frequently seen. 

 These animals are common in temperate regions 

 where they are found in fields, woods, or about 

 human habitations. They resemble spiders, but 

 differ by having the cephalothorax and abdomen 

 broadly joined and seemingly a single unit. Other 

 generally nonspider features are their compact, oval 

 bodies and their long, thin legs which enable them to 

 move rapidly. 



Harvestmen are primarily scavengers, but they do 

 prey upon small insects and suck juices from the soft 

 parts of plants. Those that act as predators kill their 

 prey be physical force alone; they lack poison glands. 

 However, phalangids do possess glands that produce 

 a malodorous secretion. These stink glands are 

 presumed to be protective, acting to repel predators. 



ORDER ACARINA (= ACARI) (Mites and Ticifs) 



Diagnosis: body oval, cephalothorax broadly 

 joined to abdomen; without visible segmentation; 



