ARACHNIDA. 



209 



dilatation which the scorpions carry at the end 

 of the tail ; it is terminated by a little sharp 

 hook generally curved backwards. Near its 

 termination there may be observed, as in the 

 mandibnlous hook of spiders, a very minute 

 orifice, or, according to some authors, two dis- 

 tinct lissnres. It is from this part that a lim- 

 pid fluid, having strongly-marked poisonous 

 qualities, exudes; and, corresponding to the fo- 

 ramen within, there is the neck of a Tittle blad- 

 der which is the true secretory organ. Little is 

 known respecting its structure: according to 

 the observations of Treviranus it is surrounded 

 by a horny substance and provided with a 

 muscle, which most probably has for its func- 

 tion the compression of the vesicle and the 

 consequent expulsion of the poison. 



Apparatus for secreting t lie fluid which con- 

 cretes in the air. This apparatus is peculiar to 

 certain arachnidans : it does not exist in the 

 scorpions nor in many other genera ; but when 

 present it is always situated at the posterior 

 part of the body. The threads by which the 

 spiders suspend themselves, and of which they 

 spin their webs, are emitted from the extre- 

 mity of the abdomen. There we find, in the 

 vicinity of the anal aperture, several small 

 appendages, which it is important not to con- 

 found with one another, (Jig. 9t3.J Of these 

 there are two which are small articulated hairy 

 Fig. 96. and filiform processes 



(b b ; )* the others are 

 spinnarets, or the or- 

 gans by which the 

 c silky threads are emit- 

 ted. Of the latter, 

 d four may generally be 

 b counted, (c d.} Their 

 structure is very re- 

 markable ; it has been 

 described by many 

 anatomists, and among others by Lyonnet in 

 his posthumous Memoirs. This patient anato- 

 mist has discovered that the surface of each of 

 the spinnarets is pierced by an infinite number 

 of minute holes, from each of which there 

 escapes as many little drops of a liquid, which, 

 becoming dry the moment it is in contact with 

 the air, forms so many delicate threads. Im- 

 mediately after the filaments have passed out 

 of the pores of the spinnaret, they unite, first 

 together, and then with those of the neighbouring 

 spinnarets to form a common thread ; so that the 

 thread of the spider, as it is employed in the 

 manufacture of the web, or such as the creature 

 suspends itself by when hanging from one's 



"* Mr. Blackball, who has published some inter- 

 esting observations on the structure and functions 

 of spiders in the third report of the British Asso- 

 ciation (1833), and more at length in a recent volume 

 of the Linnaean Transactions, considers these pro- 

 cesses to be also spinnarets. They are provided 

 with tubes, which, arranged along the under side of 

 the terminal joint, present the appearance of fine 

 hairs projecting from it at right angles ; if examined, 

 when in operation, by a powerful magnifier, the 

 function of these tubes may be ascertained without 

 difficulty, as the fine lines of silk proceeding from 

 them will be distinctly perceived. Mr. B.'s 

 observations were made on Agelena labyrinthica 

 (Walck.) ED. 



VOL. I. 



finger, is composed of an immense number of 

 minute filaments, perhaps many thousands, of 

 such extreme tenuity that the eye cannot detect 

 them, until they are all twisted together into 

 the working thread. Lyonnet has made a still 

 more curious observation: he detected in Tcgc- 

 naria civilis (Walck.) a different anatomical 

 structure of the four spinnarots. The pair 

 which is above and a little longer than the 

 other, presents on its surface a multitude of 

 small perforations, (flg- 97,J the edges of 

 which do not project, and which, therefore, 

 Fig. 97. resemble a sieve. This 



structure has also 

 been well described by 

 Leuwenhoeck, Roe- 

 sel, Treviranus, &,c. 

 The other pair, shorter 

 and lower than the 

 preceding, differs still 

 further by having pro- 

 jecting or mamillary 

 tubes independent of 

 the perforations which also exist and are analo- 

 gous to those above de cribed. The tubes are 

 hollow,and perforated at the extremity (fig. 98, a). 

 Lyonnet supposes that agglutinating threads 

 issue from these tubes, while those which are 

 Fig. 98. emitted from the per- 



forations do not poss- 

 ess that property. We 

 may observe, indeed, 

 upon throwing a lit- 

 tle dust on a spi- 

 der's web, such as the 

 circular one ofaranea 

 diadema, that it ad- 

 heres to the threads 

 which are spirally dis- 

 posed, but not to those that radiate from the 

 centre to the circumference; the latter are also 

 stronger than the others. 



. 99. Internally there 



a exists in the abdo- 



men of spiders a 

 special secretory ap- 

 paratus, which con- 

 sists of intestiniform 

 canals, united toge- 

 ther, and variable in 

 number and extent 

 according to the 

 species. In Clu- 

 bione atro.r, they 

 consist, according 

 to Treviranus, of 

 four vessels, two 

 large (fig. 99, a a) 

 and two small (b b} ; 

 near their base, and 

 not far from the 

 point where they open into the spinnarets, a 

 number of small supplementary canals (c c) 

 maybe observed. (Fig. 96 represents the spin- 

 narets in the same species.) 



Generative system. In the arachnidans the 

 sexes are placed, as in insects, in different in- 

 dividuals. It is not always an easy matter to 

 distinguish outwardly the male from the female ; 



