xi PHYLUM ARTHROPODA 627 



spinning glands open. A projection of the roof of the mouth- 

 cavity (epipharynx) is present in some Insects ; in others it is 

 replaced by a projection from the floor, the hypopharynx or lingua. 



The alimentary canal is nearly always considerably longer than 

 the body ; it is longer in vegetable-feeding than in carnivorous 

 forms. The pharynx leads into a long, narrow passage the 

 oesophagus (Figs. 530 and 531 ce.) which dilates behind into a 

 crop (in) for the storage of food. The place of this in sucking Insects 

 is taken by a stalked sac, usually termed the sucking stomach. The 

 essential processes of digestion are carried on in an elongated 

 chamber with glandular walls the stomach (cd) which may be 

 divided into several parts. Sometimes between the crop and 

 stomach is intercalated a muscular-walled chamber, frequently 

 containing chitinous teeth, the proventriculus or gizzard (pv). 

 Appended to the stomach at its anterior end are, in many Insects, 

 a varying number of tubular blind pouches, the hepatic cceca. At 

 its junction with the small intestine there open a number (from 2 

 to over 100) of narrow tubular appendages, the Malpighian tubes 

 (vm), which are the organs of renal excretion. In the cases in which 

 the development of the alimentary canal has been traced, it has 

 been found that the Malpighian tubes mark the point where the 

 mesenteron passes into the proctodseum, and it is assumed that 

 this holds good generally. The lumen of the tubes is sometimes 

 filled up with cells. In some insects, the Malpighian tubes open 

 into a paired or unpaired sac the urinary bladder. The intestine 

 is usually elongated, and its posterior portion (ed.) is dilated to 

 form a wide rectum (r.), which opens on the exterior by an anal 

 aperture situated on the ventral side of the last segment of the 

 abdomen. Anal glands (ad.), producing an odoriferous secretion, 

 often open into the rectum. 



The tracheal system (Fig. 531) communicates with the ex- 

 terior through a number of apertures the stigmata (st) which 

 vary in the details of their arrangement in the different orders. 

 They are always protected against the entry of foreign particles by 

 some means either by being surrounded by special bundles of hairs, 

 or by being provided with a special sieve-like membrane. In 

 most cases they are capable of being closed by muscular action. 

 In some Insects, mainly those adapted for active flight, such as the 

 Hymenoptera, the tracheal system is dilated in certain parts of the 

 body to form comparatively large air-sacs or air-reservoirs (tb). In 

 the aquatic larvae of some Insects there is a series of soft external, 

 simple or divided, processes the tracheal gills (Fig. 532) attached 

 to the abdominal segments and richly supplied with tracheaa, which 

 have no communication with the exterior ; in others rectal gills 

 are developed soft lamellae on the inner surface of the rectum. 



The blood-vascular system is, in comparison with the other 

 systems of organs, not very highly developed, the need of an 



