130 INTERNAL ANATOMY OF INSECTS. 
atrox) they consist of two larger and two smaller ones, 
at the base of which lie many still more minute a . The 
four larger vessels are wide in the middle, branching at 
top, and below terminating in a narrow canal leading to 
the spinnerets b . Treviranus thinks the fluid contained 
in the lower minute vessels different from that furnished 
by the larger ones — but for what purpose it is employed 
has not been ascertained. 
ii. Saliva-secretors (Sialisteria). These are organs, 
rendering a fluid to the mouth or stomach, that are found 
in many insects, especially those that take their food by 
suction, as the Hemiptera, Lepidoptera, and Diptera, 
though they are not confined to the perfect insect, being 
also in some cases visible in the larva. Swammerdam 
was one of the first that discovered them, and he suspects 
that they may be salival vessels ; though he, as well as 
Ramdohr, thinks they are the same with the silk vessels 
of the caterpillar c ; an opinion which Herold has suffi- 
ciently disproved, by showing that at one period of the 
insect's life they co-exist d , and Lyonet discovered a very 
conspicuous pair in the caterpillar of the Cossus, co-ex- 
istent with the silk-secretors e . But the physiologist who 
has given the fullest account of these organs is Ramdohr: 
— I shall therefore extract chiefly from him what I have 
further to communicate with respect to them. 
They are variously constructed blind vessels, that are 
present in almost all insects that take their food by suc- 
tion, but are mostly wanting in those that masticate it. 
They have been found, however, in Cryptorhynchus La- 
a Treviran. Arachnid. 43. t. iv.f. 42. o. p, 9. b Ibid, a, y. 
c Swamm. ii. 21. a. t. xxxvi.f. 1. abed. Ramdohr 58. 
d Schmct. t. Hi./. 1. e Lyonet—. 112. t. v.f. 1. P, Q, R, S. 
