98 GNATS OR MOSQUITOES — CHAPTER VI 



special set of muscles, by the action of which it can be 

 actively dilated. Above, it is fixed by the connective tissue 

 around it being intimately connected with that of the 

 supra-oesophageal ganglia ; and on either side are inserted 

 bands of muscular fibre, which pass obliquely forwards and 

 outwards, to their origin from the chitinous wall of the 

 ventral aspect of the occiput, and a glance at the figure 

 above will make it clear that they must necessarily exercise 

 a powerful suctorial action. 



Behind this, the pharynx once more contracts to its 

 previous diameter and just beyond the back of the head, 

 where it enters the neck of the insect ; it ends in the soft 

 oesophagus or true fore-gut, the walls of which are greatly 

 thickened at this point so as to form a bulb, rather broader 

 than that of the pharynx, and mainly composed of a 

 muscular thickening, which probably acts by way of a 

 valve, by means of which the contents of the mid gut are 

 prevented from entering the pharyngeal bulb while the 

 latter is exercising its suctorial function. It is on the 

 ventral aspect of this junction, but mainly on the oeso- 

 phageal bulb, that are placed the salivary glands which are 

 therefore contained within the narrow neck connecting the 

 head with tlie thorax. 



In the fresh insect, they extend but a little way into the 

 front of the thorax ; but in serial sections, as a result of 

 the shrinking action of the various processes to which the 

 preparation has been subjected, they are squeezed out of 

 the soft and yielding neck, back into the cellular tissue 

 filling the more rigid box of the thorax, and so are made 

 to appear to extend backwards for a considerable distance 

 into it ; and the erroneous representations that have found 

 currency have no doubt resulted from a too complete 

 reliance on the method of serial sections, unchecked by 

 dissection of the fresh insect. 



I can find no previous description of the salivary glands, 

 including that quoted in my first edition, which is not, as I 

 now find, full of inaccuracies. This has arisen from the fact 

 that their extreme delicacy renders their demonstration by 

 far the most difficult piece of minute dissection that it has 



