82 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



species, if not all of them, harbor free bacteria in their 

 intestines. It is a difficult matter to determine definitely 

 the physiological significance of bacteria in such small 

 animals. Buchner describes the free bacterial symbionts 

 in one insect, the ohve fly, {Dacus olae), which deserves 

 consideration. The description of the symbiosis in this 

 species was made by Petri ('OO), In the larva of this form 

 which feeds on the ohve, there are four large outpocketings 

 in the midgut. These pockets are lined with a flattened 

 epithelium. The lumen of the pockets is completely filled 

 with bacteria. When the larva transforms into the pupa, 

 some radical changes develop in the gut tube. With the 

 cessation of feeding, the bacteria leave the pocketsinmasses, 

 and pass through the remaining part of the midgut and the 

 hind-gut to the outside. A few bacteria remain in the 

 alimentary canal. They pass forward into the foregut. 

 During the transformations of the pupal stage, an unpaired 

 dorsal, gland-hke outpocketing develops in the pharynx. 

 In this structure the bacteria find a haven. They persist 

 in this situation in the imago, or adult. Two strains of 

 bacteria have been recognized in the diverticula of the 

 ohve fly. Paoh ('08) discovered two strains of bacteria 

 in the irregularly-arranged tubercules on the branches of 

 the ohve tree, which he thought were identical with the 

 symbionts in the ohve fly. 



There still remains a large number of symbiotic relation- 

 ships in the insects that have been described in Uterature. 

 In a few instances the symbionts are associated with dis- 

 tinct organs. For example, in certain lice, a ''mycetome," 

 richly supplied with tracheal tubes, is present in the ab- 

 domen. Certain insects harbor distinctive bacteria 

 (luminous); these cases will be left for later consideration. 



In a number of insects and insect-like organisms (ticks 

 and other arachnoids), small bacteria-Uke bodies are often 

 present. While the nature of these bodies is not definitely 



