130 



HARDWICKE'S SCIENCE-GOSSIP. 



[June 1, 1S6S. 



consider the precarious nature of their food, and the 

 long fasts which they must consequently endure, we 

 at once see the absolute necessity of some such 

 internal provision for their support. That spiders 

 are capable of existing for a long time without 

 food has been amply proved. As an instance of 

 the kind, I may cite the case mentioned by Mr. 

 Blackwali, in his admirable work on British Spiders, 

 of a female Theridion quadripunctatum which 

 lived for eighteen months in a closed bottle with- 

 out food. 



Fig. 122. Tfigenaria civilits magnified, showing alimentary 

 canal, &c, the fat-body being removed from one side. 

 a, Stomach, with caec» ; A, Intestinal canal; c, Biliary 

 organ ; d, Renal organ ; e, Fat-body. 



Embedded in the " fat-body" on each side of the 

 intestinal canal is a series of dark glandular masses 

 which communicate by means of short tubes with 

 the canal at the point of its first dilitation. This 

 is the biliary-organ. Its existence was formerly 

 denied, it having been frequently overlooked in the 

 general mass of the "fat-body." 



Also enveloped by the fatty substance, and rami- 

 fying amongst it, are two fine tubes ; these unite in 

 a small sac which communicates by a short tubule 

 with the intestine near its anal dilatation. The 

 proper function of this apparatus is somewbat 

 doubtful, but it is considered probable, and I think 

 with reason, that it is a renal organ. 



At the base of the abdomen, near its extremity, 

 there is, resting on the surface of the " fat-body " 

 a greyish-yellow mass. This when removed from 

 the spider and teased out, is found to be composed 

 of an immense number of minute tubules, ramifying 

 from a series of small glands. These tubules expand 

 several times during their course into small sacs, 

 and finally find their exit in a variable number of 

 jointed mammulse, — in the case of Tegeuaria civilis 

 six, situated externally on the inferior surface of the 

 abdomen. This is called the silk-secerning appa- 



ratus, and the external inammulse are named the 

 spinnerets. 



Each mammula is studded with a number of 

 minute orifices, through which the secretion from 

 the tubules passes. The number of these tubules 

 has been variously estimated at from four hundred 

 to a thousand. In some specimens of Tegenaria 

 civilis I examined, I found the number was about 

 seven hundred and twenty. 



It has been generally stated that the whole of the 

 mammula; are engaged in the formation of a single 

 thread of spiders' silk ; tins, however, I am very 

 much disposed to doubt, and, from what I have seen, 

 I am inclined to believe that only one or two of the 

 mammulse are in action at one time. They are 

 usually arranged in pairs, and I think it is not 

 unreasonable to suppose that two act at a time, and 

 that when the secretion supplied to them is ex- 

 hausted, the next pair come into action,, and so on 

 in succession. 



Fig. 123. Spinnerets. 



The thread is of two kinds, or rather composed 

 in two different ways, depending probably merely 

 on the volition of the animal, there being no appa- 

 rent difference in the nature of the secretion. 



The first is that which forms the longitudinal 

 threads of the web, and is simple and smooth 

 throughout its entire length; the other, which 

 forms the cross threads of the web, is studded 

 throughout by minute globules or .knobs, as it were, 

 of the silk secretion, which give it an adhesive 

 character, and doubtless serve the purpose of more 

 firmly binding together the structure of the web. 

 It has been thought that these glutinous threads are 

 spun by a special pair of spinnerets, but it appears 

 to me that the difference in the two threads is 

 caused solely by the first being spun from as much 

 of the secretion as is emitted at one time, stretched 

 to the fullest extent that its great elasticity will 

 permit ; and that the other is the result of suc- 

 cessive supplies of the fluid, each successive emis- 

 mission being marked by the formation of one of 

 the knobs or globules. 



The threads of spiders' silk are very fine, the 

 simple ones being about the thousandth of an inch 



