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Photo#740359
Ripiphoridae, dorsal - Ripiphorus

Ripiphoridae, dorsal - Ripiphorus
Plymouth Rock, Winneshiek County, Iowa, USA
July 23, 2012
Size: 3.3 and 3.8
I know, I am posting 2 individuals in the same photo. I apologize. But I want to ask a question and tell the little story.

On July 23, 2012 I was searching for aphids on goldenrod, Solidago sp. I didn't find any aphids but found other insects so collected a few. These 2 ripiphorids were dead, each in a spider's web. I stole them. (I really like these beetles.)

My question: Can anyone tell me about the size difference?

One of the other insects I collected was a leaf beetle
on which (a few days later, under the scope) I noticed a triungulin.

On July 31, 2012 I was again searching the goldenrod in the same vicinity. Again I found 2 dead ripiphorids in spider's webs. Again I stole them. Again they varied in size by .5mm.

As I continued looking, to my delight, I saw several live ripiphorids on and around the goldenrod. All told I had now seen more ripiphorids in these 2 days at this location than I had seen in my whole previous life (ok, that's not saying much)

I also collected 2 leaf beetles and a bee for the triungulin (on the previous leaf beetle) and a bit of the goldenrod. Later that day there were 10 triungulins. Perhaps they had been on the goldenrod? By evening one larva was on the ventral side of the leaf beetle and another larva was on the leg of the bee
and the rest were running around on the goldenrod.

Images of this individual: tag all
Ripiphoridae, dorsal - Ripiphorus Ripiphoridae, ventral - Ripiphorus

Size difference
At first I was thinking the size difference might be due to gender: the larger individual on the right in your photo is a female (from the monoflabellate antennae); whereas the smaller one on the left might be a male (can't tell for sure, because the antennae aren't present).

But that hypothesis doesn't jibe with the post below...with two males mounted on a female. There, the female seems intermediate in size between the middle male (largest) and the male on top (smallest)!

 

Another possibility: Sometimes adult size differences can be attributed to differences in larval food availability.

Perhaps there are other (better?) explanations?

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