ARACHKIDA. 



ARALIACK^E. 



The division furnished with air-pipes, similar to those of insects, 

 comprises Harvest- or Shepherd-Spiders (Phalangia), Mites, and 

 several other genera. " The presence of air-pipes (trachees)? says 

 M. Latreille, " excludes all complete circulation, that is, the distribu- 

 tion of blood to different parts, and its return from the respiratory 

 organs to the heart." 



The other division of the class comprises the numerous species of 

 Spiders, and the Scorpions, which M. Straus-Durckheim and Leon 

 Dufour place first. Their respiratory apparatus consists of small 

 cavities formed by the union of a great number of triangular white 

 laminae of extreme thinness. The number of these is usually two, 

 but in some species there are four, and in others eight. -The external 

 apertures of these, termed spiracles, and, as M. Latreille well remarks, 

 objectionably stigmata, are transverse chinks, corresponding in number 

 with the pulmonary pouches. 



The nervous system of the Arachnida is ganglionic, consisting of 

 nerve-knots (yanylia). In man and the larger animals, a ganglion is 

 composed of two substances similar to the cortical aud medullary sub- 

 stances of the brain, and differs from nerves in being firmer in texture, 

 and covered with a membrane of closer tissue. In the Arachnida 

 these nerve-knots are more concentrated, if the term may be used, 

 than in Insects, and they are uniform in composition, rather than a 

 chain of ganglions equally separated. Thus, in the Harvest-Spiders 

 ( Phalangia) there are a pair of nerve-knots in front of the gullet, and 

 at the back of the gullet a medullary mass, apparently consisting of 

 three ranges of nerve-knots united. 



We know nothing of the organ of hearing in A rachnida, though it 

 is certain enough that they do hear. Their eyes are all simple, not 

 composite, like those of many insects. " The eyes of spiders and 

 scorpions," says Swammerdam, " are externally formed exactly in 

 the same manner, and are smooth, glittering, and without divisions ; 

 and are as much dispersed as those that are disposed at random 

 over the body. The Wolf Spider, which catches its prey by leaping 

 on it, baa its eyes placed in the same manner." In the greater 

 number of Spiders they are 8 in number, but in some 6 (Dysdera and 

 via), and in others 2 (PhaUmyium). The arrangement of the 

 ..hen more than two, varies considerably in the different genera, 

 iiiul is taken advantage of in arranging them systematically, on the 

 principle first pointed out by Dr. Lister, and improved upon by 

 Latreille, Leach, and Walckenaer. Figures of various arrangements 

 of the eyes in spiders may be seen in 'Insect Miscellanies,' pp. 125, 

 126, after Audouin. (' Lib. of Ent. Knowledge.') 



With regard to the sexes, male spiders are always much smaller 

 than the females, being often not more than one-fourth the size. The 

 feelers (palpi), also, in the male are furnished with organs at the tip, 

 which are of various forms, but usually bulging, whereas the feelers 

 in the female gradually taper to a point. 



Looking at the size of the female spider, and the eggs which she 

 lays, it appears almost incomprehensible how they could be contained 

 in so small a body. But, by observing them more closely, it may be 

 discovered that they have not, like the eggs of birds, a hard shell. 

 but, on the contrary, are soft and compressible. Accordingly, before 

 they are laid, they lie in the egg-bag (ovarium) within the spider's 

 body, squeezed together in a flat manner ; and only come into a 

 globular form after they are laid, partly in consequence of the equal 

 pressure of the air on every side, in the same way as we see dew-drops 

 and globules of quicksilver formed from the same cause. 



The eggs of spiders, it is worthy of remark, are in most cases, 

 though not always, placed in a roundish ball ; and, as there is 

 nothing in nature without some good reason, if we can discover it, 

 we may infer that this form is designed to economise the materials 

 of tHe silken web which the mother spins around them by way of 

 protection. Whether wo are right or not in this conjecture, there 

 can be no question as to the manner in which the ball is shaped, as 

 the writer him often observed the process. The mother spider, in 

 such cases, uses her own body as a gauge to measure her work, in 

 the same way as a bird uses its body to gauge the size and form of 

 ite nest. The spider first spreads a thin coating of silk as a founda- 

 tion, taking care to have this circular by turning round its body 

 luring the process. It then, in the same manner, spins a rained 

 border round this till it takes the form of a cup, and at this stage of the 

 work it begins to lay its eggs in the cup, not only filling it with these 

 up to the brim, but piling them up above it into a rounded heap as 

 high !is the cup is deep. Here, then, is a cup full of eggs, the under 

 half covered and protected by tho silken sides of the cup, but the 

 upper still bar.;, and exposed to the air and the cold. It is now the 

 spider's t,;wk to cover these, and tho process is similar to the pre- 

 -. that is, >-l ilij. k vet) nf silk all round them, and, 



instead of a cup-shaped nest like Home, birds, the whole eggs are 

 ine|..sed in a ball much larger than the body of the spider that con- 

 structed it. 



There is a singular mechanism for the purpo. . of placing the eggs 

 in the proper position. The eggs, different from what takes place in 

 birds, are excluded from a cavity just behind the breast. Here there 

 i an organ placed somewhat in the form of a hook or a bent spatula, 

 which the spider can move in such a manner as to direct every indi- 

 vidual egg which it lays to the exact spot in the nest-cup wl'iev it. 

 wishes it to be placed. The -sense of touch hi this organ n> 



course be very acute, as by touch it must be wholly guided ; for its 

 eyes, though eight in number, and very piercing, are situated on the 

 upper part of the head, and cannot be brought within sight of the 

 nest. 



The hatching of the eggs of one species (Epeira diadema) has been 

 traced with great minuteness, and the successive evolution of the 

 embryo figured with great skill, by M. Herold of Marburg. 



M. Latreille, whose method has been generally followed both in 

 Britain and on the Continent, arranges the A rachnida into two orders, 

 as follows : 



Orders. 



Trachearia. 



Class. 



ARACHNIDA < , 



iracheaj for respiration; not moro 



(_ than four ocelli 

 The first order is divided into two families : 



( Palpi simple, pediform ; mandibles ") Families, 

 armed with a moveable and per- I 

 forated claw, emitting a poisonous j- A raneidce. 

 liquid ; abdomen inarticulate, ter- 

 minated by spinnerets . . . j 

 Palpi produced, cheliform, or shaped"] 

 like pincers; mandibles with a move- In ,. . . 

 able digit; abdomen articulate, f '"''' 

 1. without spinnerets . . . .J 

 The Araneldw include our common Spiders. [AHANEID^K.] 

 The Ptdipalpi include the Scorpions and their allies. [Scoano- 



Aracltni'1'i 

 Pulmonaria, 



The second order, Trachearia, includes very various forma, as tho 

 Pycnogonums and the Mites. [TRACHEARIA ; ACARID/E.] 



AUALIA'CE^E, Jryworts, are a small natural order of plants, nearly 

 related to the UmbeHifer(t>, from which they are solely known by their 

 young fruit consisting of more parts than two. The species of this 

 order, which includes the Ivy [HEDERA], are frequently shrubby, and 

 not uncommonly furnished with powerful hard prickles ; but they 

 are often also herbaceous and unarmed, like umbelliferous plants 

 themselves. As an illustration of the order, the American Ginseng 

 (Panax yuinrjuefvlium) may be taken. 



Ginseng (I'unar i/iiiiir/iirftiliiim). 

 1. A barren flown. 2. A fertile flower. 3. Ovnrium and styles. 



4. Fruit cut in half, with the seeds projecting. 



5. A section of n seed, showing its minute embryo. 



This plant, which is nearly related to the celebrated stimulating 

 drug called Ginseng by the Chinese [PANAX], is found occasionally 

 on the mountains of America, from Canada to tho Carolinas. It was 

 long since introduced into our gardens, but it is now seldom seen. 



