INSECTS AND THEIR NEAR RELA TIVES. 



13 



Fig. 6. — Lower side of 

 cephalothorax of a 

 spider : ntd, man- 

 dible: wjt, maxilla; 

 /, palpus ; /, lower 

 lip ; s, sternum. 



appendages — two pairs of jaws, and four pairs of legs. The 

 first pair of jaws are the mandibles (man'di-bles), the second, 

 the maxilla (max-il'lae). 



The mandibles (Fig. 6, md) lie in front of and above the 

 mouth, and consist each of two or three segments. They 

 serve for seizing prey, and often also for 

 killing it. In many books they are termed 

 the chelicerce (che-lic'e-ra;). 



The maxilla (Fig. 6, mx) lie just behind 

 the mandibles, one on each side of the 

 mouth. Each maxilla bears a large feeler 

 or palpus (Fig. 6, p). These palpi vary 

 greatly in form; frequently they resemble 

 legs ; hence many Arachnida appear to 

 have five pairs of legs. The palpi are often 

 so largely developed that each maxilla ap- 

 pears to be merely the first segment of its 

 leg-like palpus. These appendages are often 



called the/tt/z^z^/ (ped-i-pal'pi). But as the 

 name Pedipalpi is applied to one of the or- 

 ders of the Arachnida, we will call these ap- 

 pendages the palpi. 



The legs of Arachnida consist typically of 

 seven parts (Fig. 7), which are named, begin- 

 ning with the one next to the body, as fol- 

 lows : 1, coxa (cox'a) ; 2, trochanter (tro-chan'- 

 ter) ; 3, femur (fe'mur); ^patella (pa-tel'la); 

 spider. ^ tibia (tib'i-a) ; 6, metatarsus (met-a-tar'sus); 



and 7, tarsus (tar'sus). The tarsus may be composed of 

 several segments, and is usually furnished with claws. 



Two forms of breathing organs are found in this class : 

 one, tracheae, resembling the tracheae of insects, described 

 in the chapter on the anatomy of insects ; and the other, 

 tracheal lungs or lung sacs, which consist of many leaf-like 

 plates enclosed in a sac. Both forms open by paired spira- 

 cles, which are usually situated on the lower side of some of 

 the abdominal segments. 



Fig. 7.— Leg of a 



