Modern mammals are ancestrally altricial, that is, having helpless and undeveloped young. All monotremes and marsupials have small, undeveloped, essentially embryonic young that must undergo many changes until they end up resembling a miniature of the adult. Also many placentals, especially small or primitive clades, although giving birth to more developed young in relation to the aforementioned mammal groups, still have quite immature young that must undergo some crucial changes before being recognizable as miniatures of the adults of the species. All the disparate orders called insectivorans, tree shrews, most rodents, most lagomorphs, many carnivores, and surely some more orders I have missed give birth to underdeveloped, altricial young. Humans are a strange condition of secondary altriciality, so they aren’t included.
If a larva is the young stage of an animal with significant differences in morphology, physiology and behavior from the adult of the species, do altricial mammals qualify as larvae? From what I know, a larva isn’t necessary to be free-living, as many larvae are either parasitic or dependent on parental care. Moreover, it is not necessary for a larva to undergo radical metamorphosis like in holometabolous insects, as there are examples of larvae undergoing gradual change, like most salamanders and many types of fish.
What keeps us of characterizing young altricial mammals as mammalian larvae and proclaiming that mammals re-evolved a larval form? Is it due to tradition in the scientific community, the well-entrenched belief that amniotes did away with their larval stage?
I don’t know if altricial birds like passerines and parrots could be called larvae as well, as they have more abilities than altricial young mammals, e.g. feeding themselves, and the main morphological difference from adults is their lack of plumage. However it might be possible that some altricial birds qualify as larvae as well.
No. The changes are not nearly as great as animals usually considered to have larval stages.
Those changes are still far greater than in mammals, at least after birth. Salamanders undergo a change in their entire system of respiration.
Note that even in hemimetabolous insects the young are called nymphs rather than larvae. The term larva is generally restricted to holometabolous insects.
Mammalian reproduction is well advanced in evolutionary terms … there are species of organisms that have distinct and viable haploid forms in their reproductive cycles … the fern is an example … the spores that develop on the fronds will sprout into forms that are completely unrecognizable as ferns … from which the sex organs grow and “cross pollinate” to make different spores that sprout into the the more common fern form … so different are these haploid and diploid forms that they are sometimes seen as different species at first … and only later determined to be just two forms of the same species …
Mark Twain:
“How many legs does a dog have, if you call a tail a leg too?”
“Five?”
“No, four. Calling a tail a leg doesn’t make it one.”
That’s my take on it. Just because an organism looks like a little worm, doesn’t make it a larva. From what little of biology I know, the embryonic young still have pretty much the complement of organs etc. they need, just underdeveloped. Their distinctive appearance is because evolution has “chosen” to dedicate all nutrition to other purposes (more growth and development) rather than fur, or flight feathers. Note that the amount of fur or fluff is related to how likely the young will need it to survive - puppies or kittens, for example, have a modicum of fur while some others (marsupials) have a more guaranteed heat environment and don’t develop coverings early. If they were born as basically a digestive tube and rudimentary circulation system, and developed all other parts afterwards in some metamorphosis analog, maybe they would be closer to qualifying as larvae.
Perhaps the OP doesn’t really understand how radical the transformation is between “larvae” and “adult” in holometabolous insects. It’d be easy to imagine that a caterpillar is just growing some wings and a few other adult bits and pieces.
That’s not the case, though. Nearly all the larval structures dissolve during metamorphosis. Digestive tract, muscles, vascular system, peripheral nervous system, etc. turns into goo that’s only used as energy and nutrients to make the adult. All of the adult structures and organs grow are entirely new, made out of little pockets of cells that were set aside during embryogenesis. Only parts of the central nervous system are preserved.
This image shows how it works in fruit flies. Everything that’s not colored in? Soup for feeding the growth of the colored structures.
Yes I know how radical transformations are from holometabolous insect larva to adult, all the tissues dissolve and that is the reason the adult stage is smaller than the end-stage larva. That was originally the idea of a larva, but later it generalized in biological discourse. Tadpoles don’t undergo such a radical transformation, but still are considered larvae. There are salamanders that are neotenic, and apart of some rudimentary lungs, don’t change much from their larval stage. Also some aquatic frogs like pipids and the genus Lepidobatrachus do not undergo such a dramatic change. Young monotremes and marsupials aren’t unlike some larval frogs. There are many species of frog larvae that do not feed and transition to the adult based on reserves. These don’t have the extreme adaptations for feeding other species have.
On the other hand, in many supposedly little-changing hemimetabolous insects, the first nymphs or even the entire nymphal stage is quite different from the adult, for example dragon fly naiads, nymphal cicadas, and some others. So naming something a larva is both due to the observed disparity of form in relation to the adult stage and due to tradition, e.g. naiads won’t be ever called larvae because the long-established terminology describes them as nymphs, although they are quite different.
As for mammals, after a google search, I found only the Wikipedia article about larvae listing the young of monotremes and marsupials as larvae.
In older publications, this same young were called mammary fetuses. Today these seemingly larvifying proclamations are lacking, instead people have adopted more moderate terms like pouch young or puggles.
Other than that, a Google search with the terms mammal and larva in all their possible forms and combinations will mostly bring out articles about infection of young mammals with larvae of parasites or insects, something we don’t want.
It might be emotional after all. We are mammals, and we would feel strange if we are reminded that mammalian ancestors and many modern species have a larval stage. For example we see a rabbit, but it is a little strange if we imagine how it managed to survive and grow from a larval-like young. These young don’t resemble much the juvenile of the species, and have a quite alien and simplified behavior, much more strange than a kitten’s for example. They are synchronized to get the one daily nursing by the female and then they get inactive again. From my experience, I know many people creeped out by neonate rabbits and rodents. When they fluff up they are ok, but before that they are something different. Even I experienced something of the uncanny valley effect when encountering the term “mammalian larvae”.
Afterwards perhaps some people might apply the concept to human babies as well, although that is not technically correct.
It seems your question, then, is about how scientific terms originate and change. Put flippantly, scientists make shit up. They attach a mostly arbitrary name to not-yet-understood phenomenon. That name might be an analogy from common language, a description of attributes in Greek or Latin, a temporary alphanumeric designation, a dated pun that was once hilarious to some grad student, etc.
Eventually, through usage and consensus, terms can acquire very precise technical meaning or be adapted to the description of something else.
We’d need a science historian to really track the origins and evolution of the word “larva” in scientific usage. The OED says that Linnaeus originally used the term “larva” as the term for immature forms that are not recognizable as the adult (the “imago”). The term was immediately applied to insects, though I don’t know when it was first applied to amphibians.
If you wanted to drill all the way down, you could read the writings of Linnaeus and his contemporaries to find out what they were thinking when they assigned new terms to things. And then later see how the term is adapted to new things, or when there are attempts to adapt the term that don’t stick very well (like “larva” of a marsupial).
Probably, plus the assumption that our own state is the default one. Linnaeus saw that some insects developed in a very different way than we did, so he came up with a term to describe the difference.
If Linnaeus was an arthropod classifying vertebrates, he’d probably come up with a term for our freakish and awkward continual growth. Or maybe he’d say that vertebrate “adults” were just some sort of overgrown larva.
Which makes for some pretty horrific children’s books. The Very Hungry Caterpillar ends with the protagonist basically killing himself so a different creature can grow in his corpse.
I found that the original meaning of “larva” is ghost, malevolent spirit, devil, horrible mask, skeleton, etc.
Because Linnaeus used poetically many latin terms, perhaps he chose that term to describe the alien-looking immature insect stage. Now the concept of a mammalian larva becomes somewhat more terrifying.
Interesting. The term for the adult of a hemimetabolous species, “imago”, was also coined by Linnaeus. “Imago” comes from Latin, of course, and refers to the true or ideal form of the adult, as opposed to the “masked” form of the larva. But I see that in Latin, according to wiktionary, “imago” also refers to an ancestral image, or ghost.
Nah. Their basic body plan is still the same as adults. Vs. something like a beetle larvae to a beetle etc…
Something like a nymph would be closer. Grasshopper nymphs look a lot like adults.
First thought on seeing subject hed: Invasion of the Body Snatchers and all the other pod-people gestation movies. Even Alien cocoon, which implies someone/thing else’s larvae, but still counts with the people-as-larvae trapped.
I wonder why that has some kind of elemental horror to us.
You would be correct if the term ‘larva’ wasn’t broadened to encompass all differently shaped immature animals. The term ‘nymph’ nowadays is reserved only for hemimetabolous insects. Other animals that add up structures as they grow are called larvae by convention. For example amphibian larvae don’t dissolve and reorganize like beetles, yet they are called larvae. An amphibian larva looks much more like the juvenile than an insect one. The same happens with mammals, especially with monotremes and marsupials. The young are too unlike the adults, having a worm-like body and hindlimbs consisting still of buds. Even their brain has not formed the neocortex yet. Placentals are born with the neocortex though.
Also, if they were so similar to the adults, species recognition would be easy based on newborn animals. That on marsupials is quite difficult to impossible. Even in many placentals it is impossible, for example it is difficult to identify to the level of species based on the ‘pinky’ stage of muroid rodents or shrews. Ok, someone could make an educated guess based on information like locality, habitat where the newborn was found, etc. But what if you just recieve a vial with a preserved newborn inside with no accompanying information? The only solution is genetic analysis, and even that is difficult if the specimen is preserved in formalin. However the above scenario is quite improbable given that the mammalian mother or other conspecifics are usually nearby. Usually, because there are also mammals that nurse very infrequently. On the other hand, most reptiles and birds can be identified to the species level from the hatchling stage, or at least to the genus level in some cases.
When was “larva” applied to amphibians? Was it first used by Linnaeus, or someone else? How quickly was the usage adopted?
Who first used “larva” to refer to early post-natal stages of monotremes and marsupials? How many people in the field actually use “larva”? (It’s not a use I’m familiar with, but the sum of my knowledge of marsupial development comes from maybe two weeks of a comparative anatomy course…)
I don’t know and I cannot find the answer easily. Probably another doper can and or knows. But the fact is that the term ‘larvae’ for amphibians is well-entrenched today. Apart of wikipedia and some other old publications that describe young monotremes and marsupials as fetuses, I cannot find them anywhere called larvae.
