XI 



PHYLUM ARTHROPOD A 



641 



as, for example, is the case in the Day-flies (Ephemeridce). When 

 n mouth is developed, as it is in the vast majority of Insects, 

 it is situated on the lower aspect of the head, bounded in front 

 by the labrum, and behind by the labium. It leads into the 

 buccal cavity, into which open the ducts of a pair of salivary 

 glands, each of which often has associated with it a thin-walled 

 sac or salivary receptacle. Also in the neighbourhood of the mouth 

 in such larval Insects as spin a cocoon, the ducts of a pair of 

 spinning glands open. A projection 

 of the roof of the mouth-cavity (epi- 

 pharynx) is present in some Insects ; 

 in others it is replaced by a projec- 

 tion from the floor, the hypopharynx or 

 lingua. 



The alimentary canal is nearly al- 

 ways considerably longer than the 

 body ; it is longer in vegetable-feeding 

 than in carnivorous forms. The mouth 

 leads into a long, narrow passage the 

 oesophagus (Figs. 519 and 520 ?.} 

 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 pro- 

 mntriculus 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 intes- 

 tine, or further back, 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 



FIG. 519. Digestive apparatus of a 

 Beetle (Carabus auratus). 

 ad, anal glands : 06, their muscu- 

 lar appendages ; cd, stomach ; 

 trf, hind gut ; in, crop ; k, head 

 with mouth parts ; ce. esopha- 

 gus ; pv. proventriculus ; vm. 

 Malpighian tubes. (From Lang, 

 after Dufour.) 



