82 MOSQUITOES OF NORTH AMERICA 



" In bath Culicines and Anophelines there are two types of gland acinus. 

 These are recognizable both in the fresh gland and in fixed specimens. From 

 their appearance in the latter they may be termed 



(1) The granular type. 



(2) The clear or colloid-like type. 



" The Granular Type. — The greater portion of the acinus consists of cells 

 whose nucleus and protoplasm has been pushed to the outer portion of the cell 

 by a large mass of secretion which occupies almost the whole of the cell. In the 

 fresh gland this secretion appears as a clear, refractive substance, and can, by 

 pressure, be made to exude from the cell in refractive globules. In specimens 

 hardened in alcohol, this clear secretion appears as a granular mass, occupying 

 the greater portion of the cell. It stains faintly with haematein, and shows under 

 high powers a coarse reticulum and isolated globules, an appearance probably 

 due to the precipitation or coagulation of the secretion by the alcohol. 



" The protoplasm of the cell occupies, in the fully-matured gland, only the 

 extreme periphery, and the nucleus, which is much degenerated, is pushed to 

 the outer portion of the cell, and usually lies in the angular interval left at the 

 base of two or more contiguous cells. 



" The Clear or Colloid-like Type. — Of this type there is but a single acinus 

 upon either side, which usually lies between the two acini of granular type. 



" In the fresh gland the cell outlines are not so distinct as in the granular 

 type, and the secretion, when extended by pressure, is much less refractive. In 

 alcohol-hardened specimens, the acinar cells contain a large mass of clear, homo- 

 geneous secretion which, as in the last-mentioned type, fills almost the entire 

 cell, and pushes the protoplasm and nucleus to the periphery. 



" In the clear type, however, the protoplasm is always in greater amount than 

 is the case with the granular type, and the nucleus never becomes so greatly 

 degenerated. The clear, homogeneous secretion stains readily with haematein, 

 and may even stain quite deeply. With Heidenhain's hsematoxylin it frequently 

 becomes almost black. It resembles very much in appearance colloid substance 

 as it is seen in the mammalian thyroid, 



" In Anophelines this substance also distends the central duct space within 

 the acinus. In this situation an appearance is sometimes produced which re- 

 sembles faintly-stained sporozoits, but which is a normal condition. 



''' The Malpighian Tubules. — The malpighian tubules are tubular bodies with 

 caecal ends, which open into the hind-gut. The cells are extremely large, being, 

 next to the pericardial cells, the largest in the body. Each cell contains a large 

 nucleus, and contains numerous large granules which stain feebly with h^m- 

 atein, but powerfully with Heidenhain's hgematoxylin. Numerous fatty granules 

 are also present. Each cell is wrapped round a central lumen, the cells being 

 arranged alternately, so that a zig-zag appearance is given in section. The inner 

 portion of each cell is markedly striated, the lumen being thus bounded by a 

 striated area. In relation with these tubules, a large number of tracheae and 

 tracheal end-cells exist. 



" In certain conditions the malpighian tubule cells may be found quite free 

 from granules, though otherwise unchanged. This change occurs in mosquitoes 

 with large numbers of flagellates in the rectum and hind-gut. 



" The Vascular System. — The dorsal vessel is a delicate-walled tube composed 

 of longitudinal and oblique fibres with a nucleated inner layer. The fibres may 

 be traced directly from the terminations of the branched alary muscle fibres. 

 The alary fibres break up into fibres which pass in close connexion with the large 

 pericardial cells, and eventually form (1) fibres passing into the dorsal vessel 

 as longitudinal fibres, (2) fibres joining in an anastomosis in connexion with the 

 floor of the dorsal vessel. 



