March 



NA TURE 



467 



BIOLOGICAL NOTES 



The Pelagic Stages of Youxg Fishes. — The memoirs 

 by Alex. Agassiz, on the young stages of O5seou5 fishes, have 

 been noticed from time to time in our pages. In connection with 

 C. O. Whitman, he has quite recently published, as the first 

 part of vol. xiv. of the Memoirs of the Museum of Comparative 

 Zo jlogy at Harvard College, an account of the pelagic stages 

 of young fishes, which is illustrated with nineteen very beauti- 

 fuUy-exeaited plates. This memoir is devoted to descriptive 

 .sketches of the different fish-eggs which have come beneath the 

 notice of the authors, and of the early stages of the fish after 

 their escape from the egg. In a second portion of the memoir 

 it is proposed to treat of the earlier stages of cleavage, the 

 formation of the embryonic ring, the formation of the various 

 portions of the embryo, and so on. The pelagic eggs, so far as 

 the authors' experience goes, may be divided into — (i) those 

 without oil-globules, and (2) those with one or more oil-globules. 

 But this, while a convenient, is not an accurate division, because, 

 if eggs apparently without oil-globules are carefully examined, the 

 yolk-mass will be found permeated with minute fatty bodies, but 

 these are never seen to coalesce to form conspicuous oil-globules. 

 Some pelagic eggs are laid singly — this seems to be the most 

 common condition — and are left to float at the mercy of the 

 waves and winds, while others are hid in an investing gelatinous 

 mass. The single eggs seem to have the best chance of escape 

 from their numerous enemies, while as to those laid in masses 

 the chances seem all against them. When first laid, pelagic 

 eggs are usually perfectly transparent ; little by little, with the 

 formation of the embryo, chromatophores are formed, generally 

 upon the surface of the yolk-mass close to the embryo, or upon 

 the embryo itself. These chromatophores, at first colourless, 

 soon become pigmented, and while the young embryo is still 

 within the egg, the characteristic colour-pattern is often clearly 

 indicated All the eggs described in the memoir, except when 

 confined in masses, float with the embryo downwards. The 

 resorption of the yolk-m.ass differs greatly in the difterent species. 

 The rate of resorption also varies, and would seem to be corre- 

 lated with the degree of development of the structural features 

 of the embryo. The chromotaphores very rarely become den- 

 dritic before the embryo is hatched, and as a rule not till then 

 does the black eye-pigment make its appearance. The first fins 

 formed are the pectorals, traces of which appear in very early 

 stages within the egg. The closing of the blastopore and the 

 disappearance of the vesicle of Kupfter are fjUo ved by the 

 growth of the tail and the formation of an embryonic caudal 

 fin. Usually at the time of the resorption of the yolk-mass 

 the pectorals are well developed, and have become p:)werful 

 limbs ; then, too, the intestinal tract lengthens, and a swim- 

 bladder and larger alimentary canal appear. In the very 

 youngest stages, immediately on leaving the egg, the embryo 

 depends mainly upon its embryonic dorsal and ventral (its lepto- 

 cardial fin) fin for locomotion. The propelling powers are pro- 

 portionally veiy large, ani at no time of its life is the young 

 fish better provided with b:>th means of locomotion and sense- 

 organs. With growth the young fish depends, however, more 

 and more for locomotion upon the use of its pectoral fins. The 

 comparatively large size of the chorda in the ea'-lier embryonic 

 stages is a marked feature of all young fishes. The regularity 

 with which the same stages of development of identical species 

 appeared in successive years was very striking, and the authors 

 note that this regularity was not peculiar to young fishes, but 

 that they also found the spawning of the majority of marine 

 animals and their rate of development practically identical year 

 after year ; as instances of the latter they refer to embryos of 

 Agalma, Balanoglosstts, Plagusia, &c. To facilitate the identifi- 

 cation of the different pelagic stages of fish and fish-eggs, a 

 table is given, in which we find the most characteristic features 

 of the eggs and young fish so far as these have been observed, 

 the date of their occurrence, the distinctive features at time of 

 hatching, with references, so far as the forms described in this 

 memoir are concerned, to its pages and its figures, and f :ir the 

 rest to authorities where they have been described. The figures 

 on the plates are from sketches from life made by A. Agassiz 

 and Whitm.an. We trust soon to be able to notice the second 

 portion of this important memoir. 



DANAIS ARCHIPPl'S — AN ENTERPRISING BUTTERFLY. — As 

 an instance of a species extending iis geographical area under 

 our observation, perhaps there is none more remarkable than 

 the case of a beautiful and noteworthy butterfly whose natural 

 history is so pleasantly written about in the Entomologist's 



Montldy Magazine for March i886 by Mr. James J. Walker, 

 R.N. We would that our British Navy had many more such 

 excellent observers. This butterfly, which, according to the 

 rules of zoological nomenclature, is called Anosia plexippus, L., 

 is perhaps better known by its synonym, Danais archippiis, F. 

 The original home of this insect is the American continent, 

 where, including some well-marked varieties, it now enjoys 

 a vast range, extending from the Hudson's Bay Territoiy and 

 Canada to the region of the .\mazons, Bolivia, and the estuary 

 of the Rio de la Plata. Nearly everywhere through this vast 

 area it is common, and in some places it occurs in vast swarms. 

 Thus Mr. Riley says that at Missouri the air is sometimes filled 

 with these butterflies 1 1 a height of from 300 to 400 feet. These 

 swar ns appear in autumn, and some of them seem to migrate 

 southwards to warmer regions at the approach of winter. The 

 caterpillar is singularly hardy, handsome, and easy to rear ; it 

 feeds on various species of Asclepias, a genus belonging to one 

 of the most interesting families of plants known. All the species 

 of the genus are peculiar to the New World — and to the 

 northern portions of it, just that area which seems to have been 

 the birth-area of our butterfly — or to tropical Africa. Many of 

 them are hardy even in this country, and are easily increased, 

 taking care, as a gardener wottld say, of themselves. A. tuberosa 

 is a fine border plant ; A. cornuti has sweet-scented flowers ; 

 while .-;. cumssavica, though bearing showy flowers, has a foetid 

 perfume, like its congeners, the Stapelias, those African 

 Asclepiads so attractive to the carrion-flies ; this last species 

 is the one so fully described by H. Midler as fertilised through 

 the agency of Lepidoptera. The chrysalis of D. archippus is de- 

 scribed as very beautiful, being of a bright, translucent, emerald- 

 green colour, with some transverse black ridges, brilliantly- 

 gilded lines, and minute spots of a bright gold hue, and the 

 duration of the pupal condition is from fourteen to twenty days. 

 Insect-eating birds do not toach either larval or perfect forms, 

 which are free even from Ichneumon guests. It is interesting 

 to note that Mr. Riley has found the larva attacked by a Dipteron, 

 this group of insects beingintimately connected with the life-histoiy 

 of Asclepiads. The longevity of the imago is most remarkable. 

 Taking all these facts into consideration, we find this butterfly 

 well outfitted for the battle of life : the perfect insect strong 

 on the wing, with from twelve to fifteen months of a life ; eggs 

 soon hatching, after being laid on the food-plant-, these themselves 

 spreading like weeds ; larva; not molested by destructive parasites. 

 The first great march westwards was over 2350 miles to the 

 Sandwich Islands, and remembering that the appearance of the 

 insect here is subsequent to commerce with these islands, the 

 probability is that it was helped across this expanse of ocean, 

 and then it in a wonderfully steady and rapid manner spread 

 across the whole breadth of the Pacific Ocean, and far into the 

 Malay Archipelago. Carrying on its course round the world, we 

 may soon expect to hear of it in Asia. Southwards and west- 

 wards it has appeared in New Zealand and Australia. After 

 enterprise like this, it does not seem surprising that eastward it 

 should have flown to the West Indies, and at least one or two 

 exaiiples to the iVzores ; and within the last ten years it has put 

 in an appearance and been speedily captured in South Wales, 

 Devon, Isle of Wight, Dorset, Sirssex, and Kent. In 1879 a 

 specimen was even taken at La Vendee. While as garden plants 

 species of Asclepias are fairly common in Britain, in Europe 

 other food -plants might be found in I'incetoxiciiiii 0fficin.1le3.ai 

 Cynanchuiii aciitum. With such a startling phenomenon in 

 distribution, which in so gay and fine an insect cannot easily be 

 overlooked, a great light is thrown on the diffusion under perhaps 

 even more favourable circumstances as to the distribution ot 

 less conspicuous species. We should have welcomed some re- 

 marks from Mr. Walker's pen about the varieties that may have 

 arisen from the very varied surroundings that environ the butter- 

 fly ; one of these, D. enppus, seems by some entomologists 

 almost to take rank as an independent species ; that is to say, 

 that it has passed too fir away from the type to be easily recog- 

 nised as such. 



GEOGRAPHICAL AOTES 

 The following medals have been awarded by the Geographical 

 Society of Paris for 1SS6 : — The principal gold medal to MM. 

 Capello and Ivens for their three journeys across Africa ; gold 



medal to the Pandit (name not specified, but doubtless A K) 



for his journey in Tibet ; the Lagerot prize to M. Moi-sche for 

 his explorations in the Philippines ; a silver medal to M. Bloyet 



