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SCIENCE-GOSSIP. 



WHAT IS A LARVA ? 

 By H. Guard Knaggs, M.D., F.L.S. 



'1PHE term "larva" appears to be indiscrimi- 

 nately applied by batrachiologists to a tadpole, 

 and by entomologists to a caterpillar, a grub, a 

 maggot, etc. ; and it seems to me that this applica- 

 tion of the same name to two widely different phases 

 of existence is calculated to perplex the general 

 student, by leading him to infer that tadpoles and 

 caterpillars are in the corresponding stage of 

 growth. In these notes it will be my endeavour 

 to show that such an inference is fallacious. 



All animals commence life in the ovum, which, 

 with rare exceptions, comprises two distinct stages, 

 the embryo and the foetus, not to mention the 

 original germ. The chief exception to this rule 

 occurs in the anomalous family of the Amphibia, 

 where the duration of the egg state is, in many 

 cases, cut short, the condition in which the " larva " 

 makes its exit varying considerably according to 

 the species. Thus the eggs of the common frog 

 (Rana temporaria) will, in a warm climate such 

 as the south of Europe, yield forth their 

 limbless tadpoles in four days ; and when 

 it is borne in mind that impregnation has not taken 

 place until after deposition of the ova, it may be 

 guessed that the resulting larvae are in a very 

 early embryonic condition. In other cases the 

 larva makes its first appearance in a more 

 advanced state. Dr. Fritz Miiller mentions 

 ("Nature," vol. xix., p. 462) that he found a little 

 frog (Hylodes ?) bearing very large eggs, and only 

 nine in number, upon its back, and that when the 

 tadpoles came forth they were provided with two 

 legs, hind ones ; while several frogs, toads and 

 newts hatch out with four limbs, some of these 

 species being ovoviviparous, as Salamandra atra, 

 which is remarkable for the young being born with 

 gills (" Standard Natural History," Jno. Sterling 

 Kingsley. Boston, 1885), others incubating their 

 ova in pouches, or embedded in the skin, or merely 

 glued upon the backs of the females, as in Pipa 

 and Nototrema. These instances show that, even 

 amongst the Amphibia, tadpoles are by no means 

 the rule. 



Darwin (''Descent of Man," vol. i., p. 207) 

 mentions that it is well known that, at an early 

 embryonic period, both sexes possess true "male 

 and female glands, and to this statement he appends 

 the following footnote : " This is the conclusion of 

 one of the highest authorities in comparative 

 anatomy (namely, Professor Gegenbaur, ' Grund- 

 ziige der Vergleish Anat..' 1870, s. 876). The 

 result has been arrived at chiefly from the study of 

 the Amphibia ; but it appears from the researches 

 of Waldeyer (as quoted in ' Humphrey's Journal of 



Anat. and Phys.,' 1869, p. 161), that the sexual 

 organs of even the highest vertebrata are, in their 

 early condition, hermaphrodite. Similar views 

 have long been held by some authors, though, until 

 recently, not well based." 



The observations of Yung, too, who experi- 

 mented largely with tadpoles, corroborates the fact 

 that they pass through a hermaphrodite stage, and 

 he found that, when in this condition, the future sex 

 could be influenced by feeding them upon a highly 

 nutritious diet, such as beef, fish and frog meat ; 

 the latter being especially productive of a very 

 large percentage of females. Pfliiger also admits 

 the existence of a hermaphrodite stage. 



Prof. Quatrefages' " Memoirs of Man and the 

 Lower Animals," translated by Dr. Lawson, 

 observes : " The development of frogs presents 

 another curious phenomenon. It is that the young 

 animal, after it has left the egg, and before it 

 has become a larva, is still in a semi-embryonic 

 condition. At this period the digestive tube and 

 its appendages are exceedingly rudimentary. The 

 greater portion of the body is filled by a large mass of 

 yolk or vitellus, enclosed by the shin which has been 

 formed for some time, and it is at the expense of 

 this alimentary matter that the development depends.'" 

 (The italics are mine.) 



A most eminent authority upon this subject 

 writes me : — " I use the term ' larva ' to denote an 

 active embryo which has left the egg. I should 

 say, therefore, that Pipa, like Salamandra maculosa, 

 has no larval stage. Surely the young four-limbed 

 Pipa corresponds to the young four- limbed frog. 

 The changes which take place within the eggs of 

 Pipa and Salamandra seem to me to answer to 

 those which take place in the changing (develop- 

 ing) tadpole, and in that sense it may, I think, be 

 said that their tadpole state is passed within the 

 ovum ; or, if you prefer it, that part of the 

 embryonic development of Rana takes place after 

 the embryo has left the egg, and become active 

 and locomotive." 



From the foregoing notes it would seem pretty 

 clear that the larva of the frog, otherwise tad- 

 pole, is an embryo — a sort of locomotive egg, 

 minus its envelope : sexless at first, but of decided 

 sex before it reaches the equivalent of the foetal 

 stage. 



Linnaeus used the word "larva" to signify the 

 stage which masked or hid the true character of 

 the species ; and of course, in this sense, it is 

 equally applicable to the tadpole and the cater- 

 pillar ; but from an entomological point of view, it 

 conveys another and definite idea, viz. : that it is 



