Because (1) they evolved on land -- no gills, wings that are no use underwater, whole medium transfer ahead of them -- and (2) crustaceans appear in the fossil record about when trilobites do, way earlier than insects evolved, and they, being kickass creatures, have occupied the oceanic arthropod niches.
So I guess the question I should be asking is "why haven't the crustaceans taken over the land?", which they have in fact done a reasonable job at.
Interestingly, some people think that wings came from ancestral gills at some point along the line.
We have isolated crustacean homologues of two genes that have wing-specific functions in insects, pdm (nubbin) and apterous. Their expression patterns support the hypothesis that insect wings evolved from gill-like appendages that were already present in the aquatic ancestors of both crustaceans and insects.
I can't think of any insects that I know stay underwater, without coming up for air. Oh, hellgrammites I think. Other larvae? Not mosquito.
One of my college roommates used to go off on this big rant about how lobsters etc. were water insects. For some reason, it invariably made me spit whatever I was drinking on the monitor.
There is much debate over this question. Here is a website that provides simplified summaries (it's a resource for teachers) of the hypotheses for the marine insect question:
So, no, it's not entirely understood why after millions of years of adaptive radiation there are only a handful of oceanic insect species. And even still these species live on the water/air interface.
Interesting counterexamples to some of the hypotheses. The bloodworms of abyssal Baikal are way cool.
Hm, competitive exclusion is all very well when talking about a single niche and two species, but here we have a whole array of oceanic niches, and a whole array of terrestial ones, and two subphylum(?)-level groupings of species. Competitive exclusion as I understand it would have nothing to say against oceanic niches being parceled out some to crustaceans, some to insects -- freshwater niches are parceled out something like that. I can see as how exclusion might generalize to the larger scales too, but... yeah, "not entirely understood" sounds fitting.
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timeasmymeasure
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Date: 2005-08-02 03:28 am (UTC)And rainbows have nothing to hide.
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Date: 2005-08-02 03:35 am (UTC)no subject
Date: 2005-08-02 05:35 am (UTC)literal answer
Date: 2005-08-02 05:54 am (UTC)Re: literal answer
Date: 2005-08-02 07:33 pm (UTC)Interestingly, some people think that wings came from ancestral gills at some point along the line.
I can't think of any insects that I know stay underwater, without coming up for air. Oh, hellgrammites I think. Other larvae? Not mosquito.
no subject
Date: 2005-08-02 03:17 pm (UTC)no subject
Date: 2005-08-06 04:32 am (UTC)no subject
Date: 2005-08-05 11:48 am (UTC)http://entomology.unl.edu/lgh/marine_insects/marinehome.html
So, no, it's not entirely understood why after millions of years of adaptive radiation there are only a handful of oceanic insect species. And even still these species live on the water/air interface.
no subject
Date: 2005-08-06 04:31 am (UTC)Interesting counterexamples to some of the hypotheses. The bloodworms of abyssal Baikal are way cool.
Hm, competitive exclusion is all very well when talking about a single niche and two species, but here we have a whole array of oceanic niches, and a whole array of terrestial ones, and two subphylum(?)-level groupings of species. Competitive exclusion as I understand it would have nothing to say against oceanic niches being parceled out some to crustaceans, some to insects -- freshwater niches are parceled out something like that. I can see as how exclusion might generalize to the larger scales too, but... yeah, "not entirely understood" sounds fitting.