Dec. 7, 1871J 



NATURE 



lOI 



The Flight of Butterflies 



In the 103rd number of Nature there are two notices of 

 remarliable butterfly flights in America, and it is asked " Where 

 the yellow butterflies are going?" Mr. R. Spruce, in "Notes 

 on some Insect and other Migrations observed in Equatorial 

 America" (published in the Journal of the Linnean Society, 

 vol. i.";. No. 38, read June 6, 1867), has the following curious 

 account of similar flights, which, he says, have also been de- 

 scribed by Mc'Srs. Edwards, WalHce, and Bates: "The fir.t 

 time that I fell in with such a migration was in November 1S49, 

 near the mouth of the Xingii, when I was travelling up the 

 Amazon from Para to .Sau'areon. . . . We saw a vast 

 multitude of butterflies flying across the Amazon from the 

 northern to the southern side in a direction from about N. N. W. 

 to S.S.E. They were evidently in the last stage of fatigue. They 

 were all of common white and orange yellow species, such as are 

 bred in cultivated and waste grounds, and having found no 

 matrix whereon to deposit their eggs to the northward of the 

 river (the leaves proper for their purpose having probably been 

 already destroyed or at least occupied by caterpillars) were going 

 in quest of it elsewhere. The very little wind there was, blew 

 from between E. and N.E., therefore the butterflies steered their 

 course at right angles to it ; and this was the case in subsequent 

 flights I saw across the Amazon. . . . But the most notable cir- 

 cumstance is that the movement is always southward. . . . Since 

 my return to England I have read Mr. Bates's graphic description 

 of a flight of butterflies across the Amazon, below Obidos, lasting 

 for two days without intermission during daylight. These also 

 all crossed in one direction, from north to south. Nearly all 

 were species of Callidryas, the males of which species are wont 

 to resort to beaches, while the females hover on the borders of 

 the forest and depos't their eggs on low-growing, shade-loving 

 Mimosas. He adds, ' the migrating hordes, so far as I could 

 ascertain, are composed only of males.' It is possible, there- 

 fore, that in the flights witnessed by myself the individuals were 

 all males in which case the flights should probably be looked 

 upon, not as migrations, but dispersions, analogous to tho ;e of 

 male ants and bees when their occupation is done, and they are 

 roomed by the workers to banishment, which means death. In 

 the case I am about to describe, however, the swarms certainly 

 comprised both sexes, although I know not in what proportion ; 

 and their movements were more evidently dependent on the 

 failure of their food. 



" In the year 1S62 I spent some months at Chandsey, a small 

 village on the desert coast of the Pacific northward of Guayaquil, 

 where one or two smart showers are usually all the rain that lalls 

 in a year ; but that was an exceptional year, such as there had 

 not been for seventeen years before — with heavy rains all through 

 the month of March, which brought out a vigorous herbaceous 

 vegetation where almost unbroken sterility had previously pre- 

 vailed. In April swarms of butterflies and moths appeared coming 

 from the East, sucking the sweets of the newly-opened flowers, 

 and depositing their eggs on the leaves, especially of a Boerha- 

 avia and of a curious Amaranth, until the caterpillars swarmed 

 on every plant. New legions continued to pour in from the 

 East, and finding the field already occupied, launched boldly out 

 over the Pacific Ocean, as Magalhaens had done before them, 

 there to find a fate not unlike that of the adventurous navigator. 

 No better luck attended most of the offspring of their prede- 

 cessors, especially those who fed on the Boerhaavia. The shoal 

 of caterpillars advanced, continually westward, eating up what- 

 ever to them was eatable, until, on nearing the sea shore and the 

 limit of vegetation, I used to see them writhing over the burning 

 sand in convulsive haste to reach the food and shelter of some 

 Boerhaavia which had haply escaped the jaws of preceding 

 emigrants. The explanation of this continual westward move- 

 ment is not difiicult. A few leagues inland, instead of the sandy 

 coast-desert with here and there a tree, we find woods, not very 

 dense or lofty, but where there is sufficient moisture to keep 

 alive a few remnants of the above-mentioned herbs all the year 

 round, and doubtless also of the insects that feed on them. There 

 are also ca'.tle farms. When the rains come on, therefore, they 

 cause as it were a unilateral development of the vegetation from 

 the forest across the open ground, and a corresponding expansion 

 of the insect-life which breeds and feeds upon it." 



The \\ hole paper is very interesting, ijut I have copied only 

 such portions as bear on the question " Where are the yellow 

 butterflies going?" 



T. S-M. 



The Origin of Insects 



In an article by Dr. Be.ale, in your number for Nov. 23, on 

 "One of the Greitest Difficulties of Darwinism," a most extrao;- 

 dinary misconception is stated to be a difficulty. That the pupa 

 state is a modification of the ordmary process of skin-shedding in 

 the Insecta is proved by so miny facts, that one cannot under- 

 stand for a moment how it can possibly be denied, mudi less how 

 its denial can be made Uie of as an argument against the doc- 

 trine of evolution. Sir John Lubbock pointed out long ago that, 

 in the development of the Insecta, every grade of modification 

 exists between those insects which are gradually developed, 

 each successive ecdysis proiucing only the slightest possible 

 modifications, and those which undergo a change so complete 

 that it may be likened to the process of metagenesis, as it has 

 been called, which takes place in the Echinodermata. 



It is an utter mistake to suppose that any insect is redeveloped 

 during the pupa state. The most perfect instance of metamor- 

 phosis is that of the flics (some Diptera). In these the materials 

 out of which the perfect insect is developed are supplied by the 

 breaking up of the muscular system and fat bodies of the larva ; 

 but the cellular structures known as the Imaginal discs of Weis- 

 mann are formed in the egg, and persist all through the life of 

 the larva. These, it is true, only form a skin or case in which 

 the fly is developed ; but they are rea'ly nothing more than 

 a larva skin, formed on the inside of the larva skin in the e^?, 

 and detached from it by the subsequent modifications of the 

 larva. 



The nervous system undergoes extensive modification in the 

 development of the fly, but it never undergoes degeneration. The 

 mouth organs of the imago, it is true, are not the mouth organs of 

 the larva, nor are they formed by their modification, but they are 

 foreshadowed in the egg before the mouth organs of the larva 

 are formed. It is the mouth organs of the larva which are new 

 formations, not those of the imagj. In this moot extreme case, 

 the pupa skin is derive! directly from the inner layers of the first 

 larval skin, about twelve hours before the creature emerges from 

 the egg. The imaginal skin is likewise derived from cells laid 

 down in contact with the imaginal discs. There is absolutely 

 only a difference in the time at which the successive skins are 

 formed in this and in ordinary ecdysis. 



A cimex which undergoes no change of form develops each 

 successive skins from cells laid down within the last integu- 

 ment, and the same process is followed in the development of 

 the fly. 



The alimentary canal is likewise undoubtedly formed in a 

 similar manner around that of the larva, and the sexual organs 

 are gradually developed, even from the time when the embryo 

 is enclosed in the egg. 



Fritz Miiller in his "' Facts for Darwin," has shown very con- 

 clusively that the larval forms of insects are probably derived from 

 imaginal forms ; such seems, wuhout doubt, to be the case with 

 the flies {Miisca). Every day the difliculties presented by the 

 development of the Insecta to the doctrine of evolution are 

 vanishing. It is extremely probable thit insects first emerged 

 from the water with fully formed wings. We have still relics 

 of an aquatic winged insect fauna in the hymenopterous genus 

 discovered by Sir J. Lubbock. We may readily believe the 

 larval forms now existing on the earth are modified forms of 

 originally perfect insects ; we know that the larvae are subject to 

 far greater changes of life and far greater struggle for existence 

 than the perfect insects. They are all probably embryonic forms, 

 brought from the egg in a modified state before their perfect 

 development is attained. The same thing is seen in several 

 crusta;eans, which are hatched as Natiplius forms, whilst all their 

 allies attain the Zica stage in the egg. The existence of mandi- 

 bulate larva; in insects which in the perfect state have suctorial 

 mouths, is an additional argument in favour of this view. It 

 appears to be either a reversion in the larva to an anterior type, 

 for the earlier types of the Insecta were undoubtedly mandibu- 

 late, or it may by an embryonic character, which has never been 

 lost in the egg, modified by leversion or circumstances. This 

 view may appear fanciful, but the aortic arches of a fish un- 

 doubtedly exist in the mammalian embryo, and no one can say 

 what changes might take place by reversion in those arches under 

 altered conditions. Teratological embryology goes far to show 

 that the embryo may revert to long anterior types in its develop- 

 ment. 



I should, however, transgress too far on yoiu" valuable space in 

 giving proofs of all that has been put forward. I trust, how- 



