The house-fly, Musca domestica Linn: its structure, habits, development, relation to disease and control . gh the thoraco-abdominal foramen intothe abdomen to become the proximal intestine. Except in theanterior and posterior regions, where columnar cells compose thedigestive epithelium, the walls of the ventriculus are thrown intoa number of transverse folds, which are again subdivided longi-tudinally, the result being the formation of small crypts or sacculi,which are lined by large cells. These sacculi correspond to thedigestive coeca of other insects. 38 THE INTERNAL STRUCTURE OF MUSCA DOM
The house-fly, Musca domestica Linn: its structure, habits, development, relation to disease and control . gh the thoraco-abdominal foramen intothe abdomen to become the proximal intestine. Except in theanterior and posterior regions, where columnar cells compose thedigestive epithelium, the walls of the ventriculus are thrown intoa number of transverse folds, which are again subdivided longi-tudinally, the result being the formation of small crypts or sacculi,which are lined by large cells. These sacculi correspond to thedigestive coeca of other insects. 38 THE INTERNAL STRUCTURE OF MUSCA DOMESTICA The Proximal Intestine (j).int.) is the hmgest region of thegut. It varies in length considerably. In the normal-sized con-dition its course is as follows: Beginning at the anterior end ofthe abdomen, it runs dorsally beneath the heart to the posteriorregion, where it curves downwards, turns to the left, and runsforward for a short distance, curving to the right, where itdoubles back transversely to the left. Here it doubles sharplyback to the right, from whence it runs forward for a little way. Fig. 14. The posterior region of the alimentary canal, to show the rectal glands{) with their tracheal supply, the origin of the malpighian tubes {nialp.),and the position of the rectal valve indicated at X. and crosses over to the left. Curving, it runs posteriorly tobecome the distal intestine. Its walls are lined by an epitheliumof large columnar cells. The Distal Intestine (d. int.). The junction of this with theproximal intestine is marked by the entrance of the ducts ofthe malpighian tubes. It runs posteriorly and curves dorsallyand forwards to becon>e the rectum, from which it is separatedby a cone-shaped valve, the rectal valve, the position of which SALIVARY GLANDS 39 is marked externally (fig. 14, X.). The epithelium of the distalintestine consists of small cubical cells, which project into thelumen, and are covered by a fairly thick chitinous intima. Theepit
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