39G ARTHROPODA 



rangement and their jointing (fig. 429), as well as by development. They 

 are truncate and have at the tip a 'spinning field' from which numerous 

 minute, two-jointed spinning tubes, resembling hairs, arise, each of which 

 is the end of a duct of a silk gland. Different kinds of glands, producing 

 silk for different purposes, occur. The number of spinnerets varies 

 between two and three pairs, and in front of these may be an unpaired 

 spinning region, the cribrelliim, so that hundreds or even thousands 

 (Epeiridae) of glands may be present. 



The secretion of the glands hardens in contact with the air, and the single 

 threads are united by the combs of the hinder feet into a larger cord which can 

 be regulated in size according to the number of glands which are active. Yet 

 the largest cord is finer than the finest silkworm silk, hence it is often used for 

 the cross-hairs of telescopes. The spider silk has many uses; it is used to line 

 the nests, to form cocoons for the eggs, as a means of descent from high places, 

 and to form the well-known webs. 



The nervous system consists of a brain and a circumcesophageal ring, and, 

 in the Mygalidae, a single abdominal ganglion. The arrangement of the six or 

 eight ocelli and the relative lengths of the legs are matters of systematic impor- 

 tance. Two pairs of respiratory organs occur. In the Tetrapneumones there 

 are two pairs of lungs, but in the Dipneumones the hinder pair are replaced by 

 tracheae, which may open by separate spiracles (Tetrasticta) or by a common 

 opening (Tristicta, fig. 420). Rarely both lungs are replaced by trachea. 



Sub Order I. TETRAPNEUMONES. Four lungs, four spinnents and 

 eight eyes in two rows. MYGALID^;, large forms which spring upon their prey, 

 capturing even small birds and mice. To Mygale* belong the spiders (errone- 

 ously called tarantulas) which occur in banana bunches, and the trapdoor 

 spiders, Cteniza* of the southwest, which excavate burrows in the soil, line them 

 with silk, and close them with a hinged lid. Atypus.* Sub Order II. 

 DIPNEUMONES. One pair of lungs, one of trachea; at most six spinnerets. 

 Here belong most of the native and numerous tropical species. Some (VAGA- 

 BUND.E) use their webs only to line the nests and enclose the eggs, which are 

 either hidden away or carried about attached to the body; they spring upon cr 

 chase their prey. SEDENTARIA are the web builders, their webs varying 

 widely in structure. Of the first group the SALTIGRADA include forms which 

 jump upon their prey (Attus* Phidipt-us* Habrocentrum*), and the CITIGRADA 

 (Lycosa,* Dolomedes* Trochosa*), which run their prey down. Among these is 

 the true Tarantula, T. apulice of Italy, whose bite was once believed to cause a 

 frenzy only to be cured by peculiar music ('Tarantella'). The Sedentaria are 

 divided according to the web-building habits. The ORBITELARM or orb 

 weavers (Epeira* Ar slope*) form vertical webs which in many instances are 

 complete circles. The RETITELARI^: (Theridiitm* Erigone*) build irregular 

 webs. Latrodectes,* reputed poisonous to man (fig. 427). The TUBITELARI.E 

 build horizontal webs with a tube to the margin in which they lay in wait for 

 insects. 



Order II. Acarina. 



The mites, partly from parasitism, partly from other conditions of life, have 

 become, in some instances, considerably modified. With the fusion of cephalo- 

 thorax and abdomen the last traces of segmentation in the body are lost. Yet 

 they retain the six pairs of appendages four pairs of legs, which at once distin- 

 guish them from the parasitic hexapods; and two pairs of mouth parts, modified 



