202 



ARACHNIDA. 



Iron is distinctly shewn in Jig. 100, e, which 

 represents the inferior surface of the body of 

 the house-spider, (tegenaria domestica.) 



On the upper surface of the chest we rind 

 another plate much more extended than the 

 sternum, and joined anteriorly with the head 

 by means of a fissure or triangular V-shaped 

 notch which receives it. This plate or dorsal 

 shield exhibits divisions or rather lines of 

 suture which the eye readily distinguishes. 

 They represent arcs of circles arising from the 

 base of the legs and all ending in the centre 

 of the thorax, where there is a depression 

 varying as to extent and depth according to 

 the individual. In other arachnidans this 

 structure is not so clearly shewn on account 

 of the close union of the different pieces ; but 

 it is easy to detect or at least explain the un- 

 important modifications which obtain in these 

 cases. In the figure, which we have taken 

 from Savigny, of the pholcus rivulatus, the 

 traces of the division may be readily followed, 

 (Jig- 79, b.) Continued comparative researches 

 have convinced me that this dorsal plate of the 

 thorax of the araneidee is formed, not of the 

 dorsal pieces of the thorax of insects, but only 

 of the lateral pieces or those of the flancs. 

 For the arachnidans being deprived of wings, 

 the intermediate thoracic element or tergum, 

 so largely developed on account of the pre- 

 sence of those organs in the thorax of insects, 

 being no longer necessary, has completely dis- 

 appeared. How has this taken place? The 

 flancs (pleura:) which in insects were diva- 

 ricated and pushed to the sides by the tergum, 

 when that obstacle was removed, have mutu- 

 ally approximated and become united toge- 

 ther in the middle line, precisely at the place 

 where the little depression exists which we 

 have already mentioned. 



We believe that we have placed these facts 

 beyond all doubt in our ' Researches on the 

 Thorax of Articulate Animals,' presented to 

 the Academy of Sciences of Paris in 1820.* 

 Now it is worthy of remark that what has hap- 

 pened to the arachnidans, being animals de- 

 prived of wings, is also found in the crusta- 

 ceans, which are equally destitute of these 

 organs. Only that there exists in some of the 

 latter, as the decapods, a vast carapace which 

 occurs independently of the flancs, and covers 

 them. For if the carapace is raised in a crab, 

 the flancs or pleura are seen beneath, extending 

 obliquely towards one another as in the thorax 

 of a mygale, with this single difference, that 

 in the cancer, where the carapace covers the 

 flancs and protects them as well as the internal 

 soft parts, the pleitne or side pieces remain 

 divaricated and are not joined at their apices 

 as in the mygale.-}- 



9 See the Report by Cuvier, in the Analysis of 

 the Works of the Royal Academy of Sciences for 

 the year 1820. 



t We must again refer to the articles CRUSTACEA 

 and INSECTA for the full comprehension of the facts 

 which presuppose an anatomical knowledge of the 

 external covering of the animals of these two 

 classes. To-those who already possess that infor- 

 mation 1 shall observe that a single piece of the 



Digestive system. The arachnidans, whose 

 habits have been made the subject of obser- 

 vation, feed for the most part on animal matter, 

 not in a state of decomposition or even re- 

 cently dead, but in the living state. They 

 either boldly seize their prey, which consists 

 of insects of greater or less size, or they 

 attach themselves to animals much larger 

 than themselves, and live parasitically upon 

 their blood or some other nutritious fluid. 

 The latter species are generally very minute : 

 many of them, as the mites (acari), require 

 our best optical instruments for their detection. 

 The above differences in habits of life are 

 accompanied with important modifications in 

 the organs of nutrition, and especially in the 

 oral apparatus, which we proceed to de- 

 scribe. 



In the non-parasitic species, as the pulmo- 

 nary and part of the tracheary arachnidans, 

 the mouth consists essentially, first, of two 

 mandibul<E or forciples (Jig. 80, a) in close ap- 



Fig. 80. 



position, endowed with little lateral motion, 

 but rather acting vertically and provided each 

 with a hooked claw (6), which, near its point, 

 is perforated, and emits a poisonous fluid, 

 secreted by a gland, hereafter to be described. 

 In other arachnidans of the same order the 

 mandibula are a species of pincers, one nipper 

 of which is alone moveable, as in the scor- 

 pions. Secondly, of two maxilla; (c r), each 

 in the form of a more or less flattened and 

 villous lobe, provided with a palp or jointed 

 appendage (d) projecting more or less from the 

 mouth, and terminated sometimes by pincers 

 as in the scorpions, sometimes by a simple 



flancs of insects (epimera) forms the back-part of 

 the thorax of spiders ; the other piece (episternum) 

 already in a rudimentary state in the ciustaceans, 

 has completely disappeared from the thorax of the 

 arachnidans, each segment of which consequently 

 consists only of two pieces, the sternum below, the 

 epimera above. 



