102 HUGH GLASGOW. 



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 caecal bacteria. 



7. The csecal appendages themselves appear to be of profound 

 phylogenetic significance, showing a complete gradation from 

 extremely simple to very complex forms and in many cases indi- 

 cate relationships contrary to those often assumed in the arrange- 

 ment of the groups. 



INTRODUCTION. 



In certain groups of Heteroptera the alimentary canal is 

 characterized by the presence of peculiar sac-like appendages 

 which open into the mid-intestine at its extreme posteiior end. 

 These structures vary greatly in form and in degree of develop- 

 ment in the different families in which they occur, but all have 

 essentially the same histological structure and all agree in the fact 

 that they invariably contain great masses of bacteria, apparently 

 in pure culture, which are morphologically characteristic for the 

 families and often for the genera in which they occur. 



These structures were noted as early as 1809 by Treviranus 

 in a pentatomid, Pentatoma rufipes (Cimex rufipes), and again in 

 J8n by Ramdohr in Pyrrhocoris apterus as well as in representa- 

 tives of the Pentatomidse, and they were subsequently observed 

 and studied by numerous other investigators, notably by Dufour 

 in 1833. It w r as not. until about 1888, however, that Professor 

 S. A. Forbes (Forbes, '96), in the course of an elaborate series of 

 investigations on the contagious diseases of insects, discovered 

 the remarkable association existing between certain Heteroptera 

 and the bacteria which he found uniformly inhabiting the so- 

 called ca?cal glands. Leydig had previously noted the presence 

 of these organisms in the ca?cal appendages of a pentatomid in 

 1857, but he did not suggest their true nature and merely ex- 

 pressed surprise at the occurrence of such unusual structures in 

 these organs. 



The ceecal bacteria were really first observed by Professor 

 Forbes in 1882 in crushed specimens of the chinch bug, Blissiis 

 leiicopterus, and a brief technical description of the organism, 

 under the name Micrococcus insectorum, was given it by Professor 



