NO. l8 SENSE ORGANS OF COLEOPTERA — McINDOO 6l 



larva with the fingers, the hairs pierce the epidermis on the fingers 

 and are then hroken off, causing the blood of the larva to exude as 

 small drops. 



The larvae of the bean beetle are likewise covered with hairlike 

 tubercles (fig. 19, B), which bear many branches (Br), each of which 

 is terminated by a hair (Hr). While picking up the larvae the writer 

 observed a yellowish liquid on his fingers. This liquid was bitter and 

 very distasteful. After carefully examining the larvae under a hi-, 

 nocular, it was learned that the bitter liquid came from the tips of the 

 branches (C, Br). Using a needle it was possible to touch the hairs 

 (Hr) lightly, so that they broke at their weakest point ; that is, at the 

 socket which is surrounded by five processes (Pr). 



Summary 



This paper is written as a complement to the writer's (47) former 

 one entitled " Tropisms and Sense Organs of Lepidoptera," and con- 

 tains information of a similar nature, but dealing with Coleoptera 

 alone. A large mass of literature on the sense organs and tropisms 

 of beetles, including papers on light traps, attractive baits, and repel- 

 lents, has been consulted ; but only the more important information 

 found has been briefly summarized. 



The Mexican bean l)eetle was selected to represent the Coleoptera. 

 When tested to odor stimuli alone this beetle was found to be an un- 

 favorable insect ; but when the adults were allowed to come in contact 

 with the substances to be tested as foods, the beetles clearly demon- 

 strated their " likes " and " dislikes " ; and when tested to light and 

 gravity in a dark-room, the adults proved to be almost ideal for this 

 purpose. In order to obtain comparative results which could be treated 

 statistically, new technique and apparatus were devised, and the more 

 important experiments were repeated many times under controlled 

 conditions. The more important results obtained are as follows : 



When tested in a phototactic box, which lay on a table by a south 

 window in bright light, although not in direct sunshine, larvae of the 

 first and second instars were weakly photoix>sitive or indifferent to 

 light. Most of the larvae of the third instar and the more active ones 

 of the fourth instar were strongly photopositive. As a rule, the larvae 

 up to the time of pupation were fuund to be photopositive, but when 

 ready to pupate they became photonegative. Whether the negative 

 reaction is caused by a change in the structure of the ocelli is not 

 known. Hundreds of adult bean beetles were also tested and all 

 proved to be photopositive, most of them being strongly so. 



