Scale Insects

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(HOST) The males of many scale insect species come in such a mix of weird forms that science is still trying to figure them all out. Ruth Page points out that some scale insects can reproduce and some can’t; males die along with the suicidal bacteria inside them; and none of the adult males can eat.

(PAGE) Reading about the sex lives of insects always makes me feel so dizzy; it’s hard to believe we can share a common ancestor, even if that was more than three billion years ago.

Worst of all, I think, are the so-called scale insects. We’ve all seen them, looking like little bumps on plant leaves. They’re all parasites, some females so much so that they hitch onto a leaf shortly after birth and stay there till they die. That species doesn’t even have eyes or legs; all it needs is the mouth parts to suck sugars from the leaf. The symbiotic bacteria inside them probably provide protein to them in exchange for the nourishment they give the bacteria.

It’s the males of scale insects, though, that drop your jaw. Your bodily cells and mine, and those of most creatures, are pretty much clones of each other. But in scales, the cells vary a lot. The scale papas sometimes don’t contribute any of their genes to their sons; or they may give some, but there’s wide variation.

Many male offspring result from asexual reproduction, so they don’t have any fathers at all. In species that do have fathers, some never activate the male parent’s chromosomes. To top it off, in certain species, chromosomes from the father appear in some of their sons’ cells, but not in others. Much of this has been known to science for years. What hasn’t been known is, to put it briefly, how come?

It’s the bacteria inside them. These provide protein to the scales. All the bacteria come from the mother. The papa’s bacteria are no good: they’re all sterile. Bacteria in the males eventually commit suicide, and that kills the male host also. Researchers are still trying to sort out the details of all this confusion, but I don’t think we can take any more just now.

Adult males don’t look weird until you look very closely: they have only two wings, like flies, but like no other insect; they don’t have compound eyes; and they have no useful mouth parts at all, so they never eat.

The females look more peculiar but aren’t so mixed-up sexually. Females have no wings, and some may lack both eyes and legs, but they sure can eat and reproduce efficiently.

Scales suck up more sugary plant juice than they can use, so they defecate the excess. It’s called honeydew. Ants love it. Surprisingly to me, people love it, too. Natural History magazine says that, in arid regions, the dripping honeydew can leave a solid mass called manna. In fact, the manna referred to in the Bible was probably the dried excrement of scales called Tributina mannipara, which feed on tamarisk trees.

Scales are weird, but I’ll look at them with more interest now that I know how sexually mixed-up they are.

This is Ruth Page in Shelburne.

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