COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 383 



Diligent search in the streams and ponds in the vicinity of Cam- 

 bridge failing to disclose any of these animals, several dozen speci- 

 mens were procured from Ann Arbor, where Dr. Holmes obtained his, 

 and where they are often very abundant. They were shipped to 

 Cambridge in a small tin p;iil, with barely enough water to cover 

 them, but arrived in good cundition and were kept in aquaria in the 

 laboratory for a number of weeks. They were fed occasionally on 

 Asellus and whatever small water beetles or other a(iuatic insects 

 could be obtained, and when the freezing of the ponds made such 

 food difficult to obtain, it was found that mealworms (Tenebrio 

 larvae) could be substituted with apparently as good results. These 

 were offered to the Kanatras in a pair of tweezers, usually having 

 been crushed slightly in order to make it easier for the bugs to insert 

 their beaks through the hard outer covering of the larva ; otherwise 

 the Ranatras were often unable to penetrate the larvae anywhere 

 unless they chanced to find the soft integument at the bases of the 

 legs. Holmes gives, in addition to a description of their reactions to 

 light, a good account of their general habits, a knowledge of which is 

 alwaj's an invaluable preliminary to experimental work upon any 

 animal. 



Comparatively few preliminary experiments were necessary to con- 

 firm the majority of Holmes's results, including the death-feigning, 

 the head movements and swaying movements in response to light, 

 and the subsequent positive phototropism, as well as the negative 

 response to light under certain conditions. This negative response 

 was found, in part, to occur at times when there appeared to be 

 no definite assignable cause ; but in general it seemed to be due to 

 an appreciably less active condition, brought about possibly in many 

 cases by exhaustion or by lack of food. As Holmes says (:05^ p. 317): 

 " The negative reaction is associated with a condition of lowered photo- 

 tonus. It is rarely shown except when the animal is in a condition of 

 comparative sluggishness. When in great excitement, when its move- 

 ments take place with quickness and vigor, Ranatra always shows a 

 positive reaction." He found that the negative reaction usually fol- 

 lowed prolonged exposure to darkness. At times, however, tempo- 

 rary periods of negative response appear in animals which are 

 otherwise uniformly responding positively to light, under apparently 

 similar conditions and with no obvious cause. It will readily be 

 seen how this might be a disturbing influence in the experiments 

 testing the responses to both lights ; for, whereas a positive ani- 

 mal might be expected to turn toward the large light, if it dis- 

 criminated between the two lights at all, conversely an animal in a 



