28G Annals Entomological Society of America [Vol. Ill, 



tory surface. The nuclei are large, with large nucleoli and 

 coarse, deeply-staining chromatin granules, and lie in the 

 expanded inner portion of the cells. These cells are glandular 

 in nature. They stain far more intensely with haematoxylin 

 than any other cells of the alimentary canal. They arc granular 

 and finely vacuolate toward their inner margins, where a thin 

 layer of granular substance is to be seen (Fig. 18, p. m.) The 

 inner margin of this granular layer hardens to form a delicate 

 elastic peritrophic membrane which is continually fed back into 

 the mid- and hind-intestine to envelope the food material. 

 This membrane may be seen as a delicate envelope surrounding 

 the excrement as it is thrown off. In some preparations the 

 formation of the peritrophic membrane from the granular 

 secretion can be seen very distinctly with the 1-12 immersion 

 objective. 



There are several views as to the origin of the peritrophic 

 membrane in insects. By Pagenstecher ('64), it was thought 

 to be a product of the salivary glands. Frenzel ('85) thought 

 it was a product of the mid-intestinal epithelium. Simpson 

 ('00) in an unpublished paper described it as a product of the 

 mucus cells in Clisiocampa. Schneider ('87) considered \t a 

 chitinous prolongation from the esophagus. Van Gehuchten 

 ('90) and Cuenot ('95) claimed that a ring of cells just cephalad 

 of the mid-intestine produces it. In the larva of Psychoda alter- 

 nata there can be no doubt as to the origin of this membrane. 

 The girdle of cells at the anterior margin of the mid-intestine 

 secretes it. 



Behind and laterad to this girdle of cells comes a second 

 girdle of glandular cells. In longitudinal sections seldom more 

 than two cells are to be seen (Figs. 13, 15 and 4(), c. gl.) They 

 are very large, among the largest found in the alimentary canal, 

 coarsely vacuolate, and have a straw-yellow color when stained 

 with iron-haematoxylin. The nuclei are large, with rather small 

 nucleoli and fine deeply staining chromatin granules. The cells 

 ])Our their straw 3'ellow secretion, which has a uniform finely 

 granular consistency', into a pouch which opens wid.ely into the 

 lumen and extends latero-cephalad between the two rings of 

 cells as seen in longitudinal sections. This pouch-like structure 

 is simply a deep invagination on the inner surface of the gland. 



Caecae are usually present in the larvae of Diptera, espec- 

 ially in the more specialized forms. They may be long, as in 



