I understand that the strong form of Recapitulation Theory has been discredited (even as it remains a reasonable principle in weaker forms), but for insects that have a larval and then a final form, is it reasonable to assume that the larval forms resemble to some degree the ancestors (which may have lacked the mature postlarval form)?

Many arthropods have larvae, but one of their nearest relative phyla (the onychophora or velvet worms) do not.  The velvet worms do very much resemble some insect larvae. This and evidence from other animal groups suggests to some that the larval forms are closer to ancestral forms. 
But it is important to keep in mind that evolution can act on all life stages, reducing some, expanding others, and adding on at different points.

Last edited by Ajna Rivera (22nd Sep 2010 03:50:48)

Hi Pat,

As i understand it, there is still some weak form of recapitution during development, so the idea is not entirely dead, just in a much softer form than
the ideas of Haeckel, who i am led to believe wasnt entirely truthful about his data and observations in a few cases.

But, im not going to agree on this being useful to understand larval insects, as those with distinct larvae (in a group called Holometabola including beetles and flies) are a derived group, meaning they definately evolved from ancestors without distinct larval forms, the ancestors being insects similar to mayflies and dragonflies, which have juvenile nymphs, and also wingless insects like silverfish and firebrates, again with juvenile nymphs.

So i hope when you are talking about larvae, i think you mean the juvenile forms we can refer to as maggots (in some flies), or catepillars (butterflies, moths), and various other names like larvae (particularly for beetles). All these are holometabolan insects, which have a tripartite life with distict larvae-pupae-adult. There are a few exceptions, but thats it in the rough overview.   Now, its currently a bit debated which other group of insects is closest, either Paraneoptera (which includes things like aphids) or Polyneoptera (which includes things like earwigs and crickets), but it doesnt really matter. Point is these other groups of insects dont have larvae, their juvenile forms are much like the adults, just without wings, with undeveloped sexual organs, and a few other minor differences. In these other insects the juveniles are callled nymphs. Same with other definately more basal insects like dragonflies, damselflies and mayflies..as i mentioned above. etc etc.

Point is, that in all these other insects, which emerged as distinct forms earlier on in the evolution of the group, all had juveniles are more similar to the adult form, and consequently, we can be confident that the distinct larvae of holometabolan insects is derived, rather than possibly representative of the ancestral forms.

Well, if larval forms of holometabolan insects is indeed a greatly expanded developmental stage, which is indeed possible, then its an expanded stage of the very early development in their ancestors, while many later stages 'instars' that are seen in nymphs of other insects are lost. Thats a huge shift, and not totally unreasonable, but i dont think there is any real capacity for the larval stage to indicate anything like recaptulation of the ancestral forms.

Finally, a note to Ajna ... there are huge problems with relating aspects of velvet worms to the larval forms of insects. There are many other major branches of the arthropod tree between these lineages, ie chelicerates and myriapods. As you know, there is very strong evidence that 'Crustacea' are actually the sister-group to insects, so if we want to understand the early evolution of insects we should appeal to those closer relatives rather than more distant ones. Otherwise you have the danger of comming up with some absurd theory of genetic-exchange between velvet worms and insects as an 'astonishing and absurd claim', and getting ridiculed for a terrible paper in PNAS..... :)

Not to disagree with Stuart in any way, shape or form on the division between holometabolans and the rest but although things like Odonata are often termed as having an incomplete metamorphosis (egg, nymph, adult - with nymph looking like the adult form), the nymph does not hatch from the egg. The nymph form emerges only at instar two or later (depending on group). What does hatch from the egg (the first instar) is a tiny maggot-like creature. This then undergoes the first ecdysis shortly after emergence into instart 2 (which is almost always the nymph stage). So, technically, the first stage of for all those that are not holometabolans is a maggot-like creature....

To Stuart:  you caught me!  I ment to say many rather than "most."  And I was including nymphs,  nauplius larvae, and instars of other crustaceans and myriapods, though this is only the common useage of the word larvae, not a precise one.
I certainly do not want to suggest that caterpillars evolved straight from velvet worms.

Last edited by Ajna Rivera (22nd Sep 2010 03:52:07)

Well, im glad to have debate.., hopefully Ajna see's that i was just trying to make things a little clearer, and show its important to consider lots of other arthropods, as she certainly knows.

Dave has surprised me slightly, and he has spent alot more time in muddy swamps looking at larval aquatic insects than me. Honestly i can admit i never saw a first stage dragonfly, but have seen lots of other first stage insects with definate 'nymphs' as the earliest hatching stage. As ajna rightly said, evolution can act on all life stages, reducing some, expanding others. I think my point still stands that there are plenty of other insect types that arose 'after' dragonflies (like aphids, mantids, crickets) have only nymphs and no sign of a maggot-like stage.., and 'holometabolan' insects arose much later and aquired a larval form, but then holometabolan insects might be more basal than is currently assumed.... ok, time to go, i must revise some of my data...