R. W. Hegner.
The Advantage Chrysomelid Beetles Secure by Concealing Their Eggs.
Psyche 22:24-25, 1915.

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24 Psyche [February
THE ADVANTAGES CHRYSOMELID BEETLES SECURE BY CONCEALING THEIR EGGS.
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, Leptinotarsa decemlmeata, 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 PK&~ 22:24-25 (1915). hup //psyche enlclub org/22/22-024 html



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19151 Hegner-Chrysonzelid Beetles 25
as they lay in the dishes regardless of whether this was the upper or under surface in nature.
A series of experiments were then performed in order to deter- mine if direct sunlight is injurious to the developing eggs. These experiments extended over the period from June 11 to June 23, 1914. Fortunately these days were practically cloudless and thus afforded an abundant and uninterrupted supply of sunlight. The details of two experiments will suffice to indicate the nature of the experiments and the conclusions that may be drawn from them. On June 15, fifty batches of potato beetles' eggs were collected atrandom. Each batch of eggs and that part of the leaf to which it was attached was cut away from the rest of the leaf. Thirty- five batches were then pinned firmly to a board and the board was placed on top of the laboratory where the rays of the sun would strike the eggs throughout the day. The remaining fifteen batches were pinned to another board, which was placed beside the other but inverted so that the eggs were in the shade at all times. The results are recorded in Table I.
June 15
June 16
June 18
June 19
June 22
June 23
35 batches of eggs
one batch hatching (did not succeed)
one batch lost
four batches lost
none hatched
none hatched
15 batches of eggs
one batch hatched
three batches hatched
four batches hatched
five batches hatched
two batches hatched
As shown in the table all of the fifteen batches placed in the shade under the board hatched. Their ages when collected differed greatly since some of them hatched every day until all of the larvae had emerged. Those that hatched on the seventh and eighth days (June 22 and 23) must have been fresh when gathered, and their hatching must have been delayed for some reason since the normal hatching period is five to six days. Lack of moisture is



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