Large body size differences across species in Hymenoptera. Here, an example of a small masked bee (~7 mm) next to a minute ant (<3 mm) on a flower of European sea rocket (Cakile maritima). https://www.inaturalist.org/observations/120155371
And these two are both tiny next to a violet carpenter bee (Xylocopa violacea), measuring well over 40 mm in body length! https://www.inaturalist.org/observations/178658335 Similarly dramatic size differences exist within ant castes of the same species.
Wondering here out loud whether these vast differences in body size relate, at least metabolically, to the ability of hemimetabolous insects to hatch from the egg as tiny miniature versions of the adult, and then grow by moulting multiple times. I mean they must relate. These incremental steps, hormonally regulated, surely afford opportunities for evolution to meddle and trigger sexual maturation which, in needing to reallocate resources to reproduction, would concomitantly stop body growth. And it's true that smaller insects moult less times, and that moulting renders the animal helpless (and subject to e.g., predation by hornets: see this poor Mediterranean locust, Anacridium aegyptium https://www.inaturalist.org/observations/91574142 ) for tens of minutes if not more.
@albertcardona I wouldn't think it could be anything specific to hemimetabolism—the examples from Hymenoptera would show that holometabolous insects can manage great size differences.
Reproduction diverts resources otherwise devoted to wing development in ground beetles. But whether it drives body size more than foraging ability or predation avoidance, I don't know.
In my memory, beetles breeding strategy was found to play a role in species size change with increasing temperature and could predict whether species shrank or grew.
Factors such as diet and disturbance also play a role in imaginal size
The limits of growth are much higher than one would expect given the tracheal air sacks arrangement for bringing oxygen to the deep tissues. The fossil dragonfly Meganeuropsis permiana https://en.wikipedia.org/wiki/Odonata#Fossil_history measured 430 mm in body length with a 710 mm wing span. One can only wonder how many neurons this animal had, or whether its neurons were simply huge, like in Aplysia, which has rather very large synapses (e.g., https://www.jneurosci.org/content/8/7/2452.short ).
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