ARACHNIDA. 



253 



Sarcoptes Galei, or itch- 

 insect, magnified. 



or less enveloped in a sheath formed by the lower lip : the maxillary- 

 palpi are usually the only parts which have 

 free and independent movements, and their ex- 

 tremity is commonly armed either with a hook 

 or with a pair of small nippers. 



In spiders the mandibles (^fig. IM. ci) are 

 situated at the front of the head and are 

 terminated by a moveable and very sharp hook, 

 which is pierced at its extremity by a small 

 fissure, serving to give issue to the poison se- 

 creted by a gland lodged in the preceding 

 joint. The maxillae {fig, 114. h, h) are two in 

 number, and the labium {fig. 114. c) situated 

 between these organs is composed of a single 

 piece. The maxillary palpi {fig. 114. c), com- 

 pared with those of insects, are of great length 

 and size, and resemble the thoracic feet, which, in the Mygale, they 

 nearly equal in length. In female spiders they are terminated by a 

 single moveable claw: in the males the last joint {fig. 114. c?) is 

 dilated, and presents a more complicated structure. In the scorpion 

 the mandibles are short and terminate in a pair of strong pincers : the 

 maxillary palpi are proportionally more developed than in the spiders, 

 and, like the mandibles, they terminate by pincers, which are so 

 strong and large in the great scorpion {Buthus Africanus), as to 

 resemble the chelae of the Crustacea, and more especially as they 

 are succeeded by four pairs of simple and smaller thoracic legs. 



In the genus Galeodes the mandibles are chelate, but much longer 

 and larger than in the scorpions. The maxillary palpi resemble small 

 slender feet, but without the terminal hooks ; and the succeeding pair 

 of legs being similarly modified, only six ambulatory feet of the 

 ordinary structure remain. Two rudiments of antennae have been 

 noticed attached to the mandibles in certain species of this genus. 

 The head is likewise more distinct from the thorax, and it supports 

 the first of the four pairs of legs usually ascribed to the Arach- 

 nidae. These modifications, with the union of the ocelli into two 

 groups, indicate the Galeodes to form the passage to the Hexapod 

 insects. 



The modification and the connections of the pair of legs which 

 succeeds the maxillary palpi in the Galeodes, demonstrate that they 

 are the analogues of the labial palpi in Hexapod insects. The position 

 of the rudimental antennae in the same interesting genus confirms the 

 indication afforded by the nervous system in spiders and scorpions, 

 that the antennae are confluent with the mandibles, if these be not 



