





22 IV—PHENOMENA EXHIBITED BY 
means always, more beautifully coloured than the female, 
the sexual differences in this respect being sometimes very 
pronounced. An enormous amount of evidence on this 
subject is given by Darwin in his work on the ‘‘ Descent 
of Man,’’ where it is pointed out that ornamental struc- 
tures and markings have been gradually produced by the 
females always pairing with the most attractive males, the 
less attractive males thus failing to leave effspring. This 
principle is termed Sexual Selection, and although some- 
what ignored by modern naturalists, has unquestionably 
exercised a profound influence on the evolution of species. 
The courtship of insects is often a very prolonged affair, 
in which several males take part, but only one is selected, 
and it is not to be supposed that such a selection, extend- 
ing over an indefinite number of generations, has not exer- 
cised a potent influence on the race. No hypothesis apart 
from sexual selection can adequately explain the extra- 
ordinary structures and markings we observe in so many 
male insects, and it may safely be said that many struc- 
tures, at present quite inexplicable, would receive an intel- 
ligible explanation were the principle of sexual selection 
more fully taken into account. Opponents to the theory of 
sexual selection consider that an impossible amount of ¢ntel- 
ligent discrinunation is assumed on the part of the female 
insect in order that she may be competent to select the most 
beautiful, or it may be the most bizarre male. The ques- 
tion, however, is not one of intelligence at all, but of sexual 
emotion. The well-known fact that man himself is seldom 
influenced by intelligence, in matters connected with the 
mating of the sexes, should be a sufficient answer to such an 
objection. Attention was first directed by Fritz Miller 
to the remarkable scent organs present on the wings of 
many male Lepidoptera, which are unquestionably used to 
attract. the female, and have arisen through the operation 
of sexual selection. Such scent organs exist in many of our 
Native Lepidoptera; notably on vein 2 of the hind wing of 
the male of Danaida plexippus, and on the coastal fold in 
the males of Hrana graminosa and Rhapsa_ scotosialis. 
These organs usually consist of a fold, or pocket, in the 
wing, containing a large pencil of hair-like scales. The 
exact scent is sometimes difficult to detect or describe, but 
THE LEPIDOPTERA. 
an attempt should always be made to do this. In Lrana 
granunosa the perfume resembles vanilla. 
In certain species of moths the females are wholly, or 
more frequently, partially apterous. In most of those 
wholly apterous the power of walking has almost been lost 
as well as that of flight, and it was suggested. by Dr. Chap- 
man that by this means the female is compelled to deposit 
her eggs on, or in the cocoon, which appears to be always 
present in those specics having wholly apterous females. 
It is further assumed that the cocoon provides the most 
suitable nidus for the eggs and young larvae. Most of the 
species with semi-apterous females appear in winter, or 
early spring, and it is possible the abortion of the wings 
may have taken place in order to prevent the female stray- 
ing from the foodplant, and perishing from cold before 
laying her eggs.. Dr. Chapman pointed out that a winged 
female (in winter) would be unable to ‘‘scent’’ the proper 
food plant owing to the absence of any distinctive odour 
in cold weather, and considered that the loss of flight in 
winter species has been beneficial on this account. It is 
noteworthy that the foodplants of moths with apterous, or 
semi-apterous females, are common and widely distributed 
plants, so that the females’ disability to seek out the food- 
plant by flight is not a material disadvantage. 
Many species of butterflies which pass through more 
than one generation in the course of a year exhibit two dis- 
tinct varieties, the spring form and the summer form. 
This phenomenon, which is of frequent occurrence in the 
North Temperate Regions, is called seasonal dimorphism. 
The summer form is supposed to have originated when the 
climate ameliorated at the close of the last glacial epoch, 
the spring form representing the species as it existed during 
the cold period, when the summer only allowed time for the 
production of a single brood in the year. Similar 
dimorphism exists in the tropies in connection with wet and 
dry season forms. No instances of seasonal dimorphism 
have been observed in New Zealand at present, but the 
question is of special interest as it may throw some light 
on the existence of a glacial period in this part of the 
world. 

