The Gastric Cæca and the Cæcal Bacteria of the Heteroptera

Certain of the Heteroptera are almost unique among insects in the possession of well-developed cæcal appendages situated at the extreme posterior end of the digestive portion of the gut. 2. The cæca are remarkable in that wherever they occur they are invariably filled with bacteria. 3. These bacteria are not only always present in the cæca of normal bugs but the association is hereditary, the organisms appearing early in the alimentary canal of the developing embryo. 4. Structurally the bacteria from different hosts vary greatly, ranging from minute, coccus-like bacilli measuring often less than one micron, to huge, spirochæte-like forms thirty microns and more in length; but in whatever form they occur they are morphologically characteristic for the particular species harboring them. 5. That these are unquestionably bacteria is shown conclusively by culture experiments checked by agglutination tests. 6. These normal bacteria appear not only to inhibit the development of foreign bacteria but to exclude them altogether, the mid-intestine being normally wholly free from the invading bacteria and protozoa which are common in many related insects and this is probably the chief function in the life processess of the host performed by these cæcal bacteria. 7. The cæcal appendages themselves appear to be of profound phylogenetic significance, showing a complete gradation from extremely simple to very complex forms and in many cases indicate relationships contrary to those often assumed in the arrangement of the groups.