600 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 67 1 . 



ice made in the cane was heavier in specie than that in the air : which he dis- 

 covered by putting it into a fluid, which was in specie hghter than water, but 

 heavier than ice made in the open air. 



Of a Kind of viviparous Fly ; together ivitJi some Inquiries about Spiders y 

 and a Table of 33 Sor^ts to be found in England. By Mr. Martin 

 Lister. N'' 72, p. 2170. 



Sir — I return you thanks for your obliging letter of the third of January, and 

 have sent you the viviparous fly, and the set of inquiries you desire of me. The 

 fly is one of the largest of the harmless tribe that 1 have met with in England. 

 I call them harmless, because they are without that hard tongue or sting in the 

 mouth, with which the oestrus kind, or gadflies, offend both man and beasts. 

 This fly is striped upon the shoulders grey and black, and chequered on the tail 

 with the same colours : the female may be known by a redness on the very point 

 of the tail. The very latter end of May l666, I opened several of them, and 

 found two bags of live white worms of a long and round shape, with black 

 heads ; they moved both in my hand and in the unopened vesicles, backwards 

 and forwards, as being all disposed in the cells along the body of the female like 

 a sheaf. 



This is the only fly I have observed with live and moving worms in the belly 

 of it ; yet I guess, we may venture to suspect all of this tribe to be in some 

 measure viviparous.* 



Then follow some general inquiries -{- concerning Spiders, which are omitted, 



* The circumstance here described is obser\'ed to take place in more than one kind of fly : the 

 eggs, in such species, hatching internally. 



f The natural history of spiders was but obscurely known in the days of Lister : it is however so 

 well detailed in the works of Clerk, Roesel, &c. tliat the queries here proposed may be considered 

 as useless. It may be therefore more proper to give a few of tlie leading particulars in the oeconomy 

 of tliose insects, than to reprint the above-mentioned inquiries in the works of Dr. Lister. 



1. Spiders are hatched from eggs deposited by tlie parent animals, and are excluded from the egg 

 foil formed, and without undergoing any farther change than a gradual increase of size, and greater 

 or less varieties of colour after each period of casting their skins, which are deposited in such a man- 

 ner as to appear perfectly complete, so as to represent the insect itself, with the difference only of a 

 greater degree of transparency, and a more contracted or shrunk abdomen. 



2. In feeding, the spider not only sucks out the juices of its prey, but also occasionally devours or 

 comminutes with its jaws some of the more solid parts. (See Clerk's Aran. Suec.) 



3. The glutinous fluid or substance of which the thread or web of spiders is drawn, is contained 

 in a pair of undulated receptacles in the abdomen ; and the animal possesses the power of forcibly 

 evacuating or propelling this gluten occasionally to some considerable distance : this is termed the 

 darting or shooting of the thread, and appears to have been known to Aristotle and Democritus. 



