PHYLUM ARTHROPODA. SPIDERS AND THEIR ALLIES 



277 



sheep; it causes intense irritation, loss of 

 wool, and decreased vitality. Unless these 

 animals are properly treated, they may die. 

 Several important vectors of disease oc- 

 cur among the ticks. Rocky Mountain spot- 

 ted fever of man is due to Dermacentor, 

 which transmits germs (rickettsial organ- 

 isms) from rodents and larger mammals to 

 man; the female ticks pass the disease germs 

 to their offspring through the egg. One 

 symptom of this disease is the appearance of 

 spots on the wrists, ankles, trunk, and face. 

 An eastern variety of spotted fever occurs 

 in country districts from New York to Flor- 

 ida; cases have also been found in the Mid- 

 dle West. A vaccine has been developed 

 that confers immunity for about one year. 

 The Rocky Mountain wood tick is also re- 

 sponsible for tick paralysis, which appears to 

 be due to a poisonous salivary secretion 

 (toxin) injected by the tick, and which may 

 prove fatal to children. Another disease 

 known to be transmitted by ticks as well as 

 by certain insects (Fig. 160) is rabbit fever, 

 or tularemia. Relapsing fever, characterized 

 by alternating periods of fever and normal 

 temperatures, occurs in various parts of the 

 world, including Texas, Kansas, Montana, 

 Utah, and California. Apparently a different 

 species of tick of the genus Ornithodoros 

 transmits the disease germs (spirochetes) in 

 each locality. Texas fever in cattle is trans- 

 mitted from diseased animals to healthy 

 ones by the cattle tick (Boophilus) (Fig. 

 167) which passes the infective agent (spor- 

 ozoan) to her offspring through the egg. 

 Among the continents of the world, Africa 

 b especially burdened with more than its 

 share of different kinds of ticks. 



CLASSIFICATION OF 

 THE ARACHNOIDEA 



(For reference purposes only) 



Class Arachnoidea. No antennae; no true 

 mandibles; body usually of 2 divisions, cephalo- 

 thorax and abdomen; 6 pairs of appendages on 

 cephalothorax; first pair of appendages are 



nippers, termed chelicerae; 1 pair of pedipalpi 

 variously modified; and 4 pairs of legs. 



Order 1. Xiphosura (Or. xiphos, sword; 

 oura, tail). King or horseshoe 

 crabs (Fig. 166). Crablike; 

 cephalothorax horseshoe-shaped; 

 tail or telson long and spikelike. 

 Ex. Limulus polyphemus (for- 

 merly Xiphosura). 

 Subclass 1. Arachnida (Gr. arachne, spider) . 

 Spiders, scorpions, mites, etc. 

 No antennae; no true mandi- 

 bles; first pair of appendages 

 are chelicerae; cephalothorax 

 and abdomen usually evident. 



Order 1. Scorpionida (L. scorpio, scor- 

 pion). Scorpions (Fig. 166). 

 Elongated; long abdomen of 1 3 

 segments; sting at end of 

 tail. Ex. Centruroides * gertsch, 

 Arizona. 



Order 2. Pedipalpi (L. pes, foot; palpo, 

 touch gently). Whip scorpions, 

 etc. (Fig. 161). Pedipalps thick 

 and strong; first pair walking 

 legs with many-jointed tactile 

 flagellum. Ex. Mastigoproctus 

 giganteus, the garoon. 



Order 3. Araneae. Spiders (Fig. 164). 

 Cephalothorax distinct; abdo- 

 men usually unsegmcnted; 

 cephalothorax and abdomen 

 joined by a narrow waist; cheli- 

 cerae small, a poison duct in 

 the terminal fang; book lungs, 

 some with tracheae; spinnerets 

 on abdomen; chiefly terrestrial. 

 Some of the common families 

 are as follows: 



1. Theraphosidae. Tarantulas. Large, hair)', 

 hunting spiders, with 2 pairs of lungs, con- 

 fined to the warmer regions of the globe, 

 and hiding in holes in the ground or in 

 crevices on tree trunks, etc. Sometimes 

 brought to northern cities on bunches of 

 bananas. Ex. Dugesiella hentzi of our 

 south and southwest. 



2. Ctenizidae. Trap-door spiders. Moderately 

 large spiders with 2 pairs of lungs and 

 with stout strong legs. They live in under- 



* Formerly called Centrums. 



