MICROSCOPIC OBJECTS. 84 



Fig. 167. House-Spider (Aranea domestica), x 3. 

 Class Arachnida. 



The Class Arachnida, which comprises the Spiders, Mites, 

 &c., does not belong to the Class Insecta proper, but is placed 

 between them and the higher articulated animals. The de- 

 scription of the order is as follows : Head united with the 

 thorax, forming a cephalothorax ; antennae none ; eyes simple 

 (ocelli) ; legs eight, jointed. The organs of the mouth vary 

 according to the families; in the Araneida, or the true Spiders, 

 they chiefly consist of mandibles (see fig. 168), and in the 

 Mites they often terminate in a bifid labium. Spiders may 

 be mounted in the same manner as any whole insect, viz. 

 with a fine needle prick a small hole in the abdomen, then 

 immerse in liquid potassa fusa for a few hours or days accord- 

 ing to size, next press gently between glass, replace in the 

 potassa for an hour or so, then press the rest of the matter 

 from the body. Well wash in warm water, dry under pres- 

 sure, soak in turpentine, or distilled Canada balsam, until 

 transparent ; finally mount in balsam as usual. 



Fig. 168. Mandibles of the House-Spider (Aranea 

 domestica) , x 20. 



The mandibles as mentioned at fig. 167, comprise the chief 

 part of the mouth of a Spider ; they are the two claw-shaped 

 bodies in the centre of the drawing ; each claw is pierced nearly 

 through so as to admit of a poisonous secretion passing from 

 the gland situated at its base up the canal and through the 

 small hole near its apex into the body of the insect attacked, 

 by a similar action to that of the sting of the nettle (see 

 fig. 35). On each side of these mandibles are situated the 

 maxillary palpi, and at their base are the maxillae; lastly 

 between these is the labium. 



The method of preparation is the same as mentioned for 

 parts of insects (see figs. 141, 155, &c.). 



