100 GNATS OK MOSQUITOES — CHAPTER VI 



These are the blind ends of the lobes of the salivary 

 glands, of which there are six in all ; each gland consisting 

 of three sausage-shaped lobes of nearly equal size ; but as a 

 rule only two or three of their number will come into view 

 at this stage and their subsequent separation from the 

 structures surrounding them is purely a matter of manipula- 

 tion, for which I can offer no better receipt than patience and 

 steady hands. Once one has learned to recognise their very 

 characteristic appearance, there is little difficulty in getting 

 piecemeal preparations, but I was well nigh in despair ere I 

 had cleared up my last doubts as to their exact form and 

 connections. 



In structure these glands differ entirely from any with 

 which the student of vertebrate histology is familiar, as, 

 strictly speaking, each cell is a separate gland, there being 

 no true duct formed by a lumen bounded by contiguous 

 gland cells and supported by a fibrous layer, such as is 

 typical of vertebrate glands, as well as those that appertain to 

 the intestine proper in arthropods. Instead of this, the 

 sahvary cells are arranged, like the flowers of a " spike," on 

 a chitinous tube prolonged from the lining of the buccal 

 cavity, and the singularly sharp outline and appearance of 

 rigidity of this tube, form a marked contrast with the 

 indeterminate appearance of the lumen of an ordinary 

 tubular gland. Apart from the fact that it is quite straight, 

 the intraglandular portion of this chitinous duct resembles 

 nothing so much as an elastic fibre from the ligamentuvi 

 nuchcB, but is of much less diameter. The external diameter 

 of even the main duct is only 3 or 4 /iz, and the lumen of its 

 intraglandular branches can hardly exceed 0"5yLt. Each lobe 

 of the gland consists of a single branch of the duct with the 

 salivary cells closely clustered round it, but in no other way 

 connected with each other, so that the least touch or the 

 pressure of a cover glass, at once separates them from each 

 other and from the duct, to which each cell is connected 

 only by a delicate thread of cell substance prolonged from 

 their smaller ends. Each is a typical gland cell of con- 

 siderable size and pear-shaped form, with large nucleus and 

 nucleolus, the smaller end tapering off to a delicate thread 



