626 ARACHNI1>A. 



in its form approaches the spiders, as noticed by Latreille. 

 The four-jointed antennse are long and elbowed, while there 

 are eight simple eyes on each side of the head. The species 

 are found on the leaves of garden plants. In Papirius of 

 Lubbock, the antennae are said to be "four-jointed, but with- 

 out a well marked elbow, and with a short terminal segment, 

 offering the appearance of being many-jointed. * 



SUB-CLASS IL AKACHNIDA. 



The t^'pical forms of this order have the body divided into 

 two regions, the head-thorax (cephalothorax) and abdomen. 

 The head is sometimes quite distinct, but is generally sunken 

 into the thorax, which bears four pairs of legs, while the abdo- 

 men has no organs of locomotion, though the abdomen is pro> 

 vided with three pairs of jointed appendages (the spinnerets), 

 which are, however, homologous with the legs. The metamor- 

 phosis is very incomplete in the lower forms, while in the 

 spiders there is none at all after the animal leaves the egg. 

 The head is without antennae, or compound eyes. The order 

 shows some analogy with certain Dipterous insects, especi- 

 ally when compared with the wingless Chionea and Nycteribia, 

 and its lowest forms (certain mites) bear a close resemblance 

 to some of the lower Crustacea, as the young stages and em- 

 bryonic development are remarkably similar. The typical 

 forms of flie order homologize too closely with the apterous 

 insects to allow them to be separated as a distinct class. We 

 shall see below that the rank here assigned to the group ac- 

 cords well with their anatomical characters and habits. 



In some genera there is a decided line of demarcation 

 between the head and the thorax, which is, however, ^ery 

 distinct during embryonic life, and we do not perceive that 

 gradual transition from month-parts to swimming legs which 

 obtains in the Crustacea. The oi'der, however, has much 

 lower, more degraded forms than the Myriopods even, as 

 the genus Demodex testifies, wIul-Ii may recall readily certain 

 intestinal worms. This we would consider as but an example 



* Explanation of Plate 10. — Fig. 1, Lepisma saccharina Linn?;" Fig. 2, 3 

 Degeeria flavocincta Pack.; Fig. 4, 5, D. purpurascens Pacli. ; Fig. 6, 7, Isotoma 

 idumbea Pack.; Fig. 8, 9, Machilis viri-ddUs Say. 



