GASTRIC C.ECA OF THE HETEROPTERA. 113 



metabolism of the host, do not represent the typical organism 

 any more than the Leptus irritans stage could be taken as repre- 

 senting the true life cycle of the particular species of Trom- 

 bidiitm to \vhich it belongs. The zoochlorellae, like Leptus 

 irritans, really represent an abnormal departure from the regular 

 development of the species. The form to which these symbiotic 

 algae belong is really a typical, free-living, flagellated organism, 

 which is normally free during its entire, regular life cycle, the 

 zoochlorella stage representing merely those individuals which 

 chance to be swallowed by the worm. The free-living stage of 

 this organism will grow vigorously on artificial media and infec- 

 tion is found to take place readily from pure cultures, but in the 

 case of the organism direct from the tissues of the host worm it 

 has been found not only that they will not develop on artificial 

 media, but that they have been so modified by their short exist- 

 ence in the body of the host, that they are not even able to set 

 up the infection in clean worms although they may be swallowed 

 in quantities. 



In certain coelenterates the relation of the alga to its host 

 animal appears to be better established than in the planarians. 

 In Millepora (Mangan, '09), at least, the algae are found in the 

 egg and appear to be transmitted through it for at least one 

 generation; although there is a possibility that the inclusion of 

 these organisms in the egg is a mere accident and that the 

 chief source of infection is the free-living stage of the alga which 

 is continually being swallowed by the animal. 



The only other well-known bacterial infection in insects directly 

 comparable with that in the Heteroptera is one whose relations 

 have been worked out by Petri for the little olive fly, Dacus 

 olecB a subject which will be discussed in detail in another place. 

 In this insect the bacteria, which are also intestinal forms, are 

 present in the larva as well as in the adult and, according to 

 Petri, there is a complicated modification of the ovipositor which 

 functions as a secondary reservoir for the intestinal bacillus. 

 Petri at first very naturally reasoned, since these bacteria were 

 present in larvae that had hatched from eggs laid by sterile flies 

 in sterile olives, that the organism must be transmitted through 

 the egg; but in his later work he concludes that this is not the 



