A select few

It’s true; only a few special people would not only notice details like this, but photograph it in fine detail. I’ll let you supply your own definition of ‘special’ for these circumstances…

In clearing out some areas of the yard the other day, I disturbed some wet leaves and wood pulp and exposed my friend here, obviously suited to a hidden and protected existence. I had to include a scale shot – this is easily the biggest grub I’ve ever seen, and my best guess is this being a patent-leather beetle (Odontotaenius disjunctus,) judging from the size and the habitat. But the size isn’t the most interesting bit.

scale photo of grub, possibly patent-leather beetle Odontotaenius disjunctus
I don’t know if this image really conveys the correct impression or not (don’t worry – I’m on it,) but the grub was both seriously distended and almost completely transparent. This allowed me to illustrate two particular facets, which of course I will inflict upon present to you.

large grub possibly patent leather beetle Odontotaenius disjunctus showing internal anatomy
The entire hind-end of the abdomen, taking up 1/3 the mass of the whole insect, was engorged with a brown mass that was likely wood pulp under digestion, but resembled brains more than anything else, especially due to the bi-lateral ‘lobes’ appearance. If I’m correct in my assessment, this would seem to indicate that the digestive tract of the beetle (?) is bifurcated, and thus significantly different from our own – no surprise there of course, but it’s little reminders like this that carry us away from the assumptions of similarity that we’re prone to make. And while I’m here I have to say that, as I was getting these photos, the grub would perform an occasional ‘contraction’ and the brown mass would shift under the skin, and not all at once either, but in different sections. There’s something very different about seeing working internal anatomy.

It’s easy to think that the lobed appearance might indicate lungs or something, but that’s completely on the wrong track, and the second facet that is being illustrated. As I’ve posted about twice before, here and here, arthropods have an entirely different respiratory system. In most cases, they have little openings along the thorax and abdomen, called spiracles, that feed into a branching network of tracheoles that carry oxygen to the tissues directly, without having to use a circulatory system like mammals do. I was lucky enough to find some caterpillars that showed this visibly through their skin (the first link above,) but this guy didn’t even require magnification – the networks were easily visible by eye. And in fact, so visible that, under magnification, the bigger branches actually had a translucent appearance.

visible tracheole network of grub possibly patent-leather beetle Odontotaenius disjunctus
[Yes, there’s a little piece of bright blue fiber next to a rust-orange spiracle that looks like somebody was being careless with a pen – don’t ask me where it came from, and I didn’t see it when taking the photos.]

There is no muscular mechanism for pumping air in and out of the body for insects, or at least the majority of them – it just comes in largely through the flexing of the abdomen and internal organs. Arthropods don’t actually need much oxygen, both because of their size and because their metabolism doesn’t require it as a catalyst as much as mammals do – a little goes a long way. This is why exterminators that treat a house must evacuate everyone and seal it up, since the concentrations of pesticide that are required are a lot higher than what would affect us, weaklings that we are.

The spiracles themselves were also big, larger than I’ve seen on any arthropod, and peculiarly-shaped – right now I could only speculate as to why, so we’ll just leave my ignorance sitting quietly in a corner rather than parading it around for all to see.

spiracle detail for grub, possibly patent-leather beetle Odontotaenius disjunctus
The exoskeleton around the spiracles seemed to take on a thicker and more opaque appearance, giving the impression of being ‘reinforced’ there for some reason, but on very close inspection it looked more like a spray of tiny tracheole to feed the closest tissue. Where would you be without people like me to present the wonders of the arthropod world to you? Probably sleeping better at night, is my guess…

While those chelicerae (‘fangs,’ ‘jaws,’ whatever you like) in the top photo look capable of dealing a serious pinch at least, I never encountered them in that manner, and the grub stayed mostly curled up tight in a defensive posture, not helped at all by me shifting it to a better angle for images every time it decided it was safe and started unrolling again. After these photos, and grossing out The Girlfriend, I returned it to where I had found it. It might have been interesting to see what it eventually morphed into as an adult, but I don’t really have any method of housing it for such. And I think it was actually too big to consider feeding to the resident green frogs, who seem to be keeping the spider populations in the yard down anyway.

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