24 Psyche [February 



THE ADVANTAGES CHRYSOMELID BEETLES SECURE 

 BY CONCEALING THEIR EGGS. 



By Robert W. Hegner, 



Zoological Laboratory of the University of Michigan, Ann Arbor, 



Michigan. 



It is obvious to everyone that eggs that are concealed will be 

 more likely to escape destruction than those that are laid in open 

 situations and hence easily seen by predatory animals. There 

 is good reason for believing, however, that in the case of those 

 chrysomelid beetles that lay their eggs on the under surface of 

 leaves, there are other factors involved which make concealment 

 only one of the advantages thus secured. It is the general habit 

 of insects as well as many other animals to hide their eggs. Often 

 this results in their development in the dark. Large numbers of 

 animal eggs develop normally in the dark since those that pass 

 through the embryonic period within the body of an opaque parent 

 or are surrounded by opaque envelopes are effectively shielded 

 from the light. Many experiments have been performed with 

 the eggs of a large number of animals in order to test the influence 

 of darkness upon embryonic development, but the general con- 

 clusion has been reached that the lack of light neither retards 

 nor accelerates growth. In beetles' eggs the chorion allows the 

 penetration of light rays and hence the direct rays of the sun must 

 reach the interior of the eggs exposed to them. 



So far as I know no one has determined the reasons why many 

 insects lay their eggs on the under surface of the leaves of plants, 

 rather than on the upper surface. The experiments described in 

 this paper were undertaken with the idea of furnishing data which 

 might assist in explaining this phenomenon. The potato beetle, 

 Lejptinotarsa decemlineata, was found to be favorable for this pur- 

 pose because of the ease of obtaining its eggs and of rearing it in 

 the laboratory. Besides the normal development of the eggs of 

 this beetle is well known. 



It was first proved that there is no difference between the upper 

 and lower surfaces of a potato leaf which causes the beetles to pre- 

 fer the lower surface, since individuals placed in stender dishes 

 in the laboratory laid their eggs on the under surface of the leaves 



