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SUMMARY OF CURRENT RESEARCHES RELATING TO 
observed in Eristalis, Libellula, and Stenobothrus. The varying influences 
of light and darkness are not manifested equally in the eyes of all Insects, 
for in some the changes are scarcely noticeable, while in others they are 
strongly marked. 
When we consider the universal presence of pigment in the eyes of 
all animals, even the simplest, we cannot fail to see that it plays an 
important part in the physiology of vision. As it is now known that 
the eyes of both Vertebrates and Arthropods possess the power of 
adapting the arrangement of the pigment to the quantity of light, we 
shall not be astonished if it is discovered that the eyes of other animals 
possess a similar power. 
a. Insecta. 
Power of Sight of Insects.* — From observations made on the visits 
of insects to flowers, Herren W. O. Focke and E. Lemmermann conclude 
that Lepidoptera and Diptera are in many cases attracted to flowers 
chiefly by the sense of smell ; while w 7 ith Hymenoptera this is much more 
rarely the case, but occurs in the lime. Insects see clearly in only the 
immediate neighbourhood of the object ; with Apidae the impression 
becomes indistinct at a distance of ten cm., and many Lepidoptera and 
Diptera are even more shortsighted. More distant objects convey to 
insects only a very indistinct visual impression ; but differences of 
colour can be perceived from a comparatively great distance. A brightly 
coloured flower one cm. in diameter in green foliage can be seen by 
Apidae and Lepidoptera at a distance of 1-2 metres. The perception 
of colour in insects is developed in very different degrees, and in different 
directions in different species. 
Formation of the Dorsal Region in the Embryos of Insects.f — 
Dr. J. Nusbaum adds to the five different ways in which Graber showed 
that the dorsal region might arise, a sixth observed in the embryo of 
Meloe. The entopygma ruptures on the ninth day of development, the 
ectopygma not before the nineteenth day. The ruptured portions of 
the entopygma, connected with the ectoderm, lie midway between the 
dorsal and the ventral surfaces of the embryo, and unite with the wall 
of the still unruptured ectopygma. Thereafter the ectopygma is ruptured 
below the point of union, the whole of the lower or ventral portion 
gradually degenerates, but the dorsal portion with the united part of 
the entopygma contracts to form the dorsal wall of the embryo. The 
final boundary of the back, however, is wholly due to the entopygma, 
for the implicated portion of the ectopygma is invaginated into the yolk 
as a “ dorsal tube,” the cells of which are soon scattered in the yolk, 
and take no direct share in building up the embryo. Unlike Graber, 
Nusbaum maintains that differences in the relative quantity of nutritive 
yolk, in its consistence, and in the duration of development, influence 
the mode in which the dorsal region is formed. He regards the 
differences in the development of this region as in many respects 
cenogenetic, and would seek to associate them with the special adapta- 
tions of the embryo and larva to various conditions of life. 
* Abhandl. Naturw. Yer. Bremen, xi. (1890) pp. 439-43. See Bot. Cenlralbl., 
xliii. (1890) p. 36. t Biol. Centralbl., x. (1890) pp. 110-4. 
