Tuesday, December 31, 2013

Winter invertebrates

It is officially winter, the Northern Hemisphere has had its shortest day/longest night, and now our days will begin lengthening.  Our good friends who live in northern Sweden have just written to us that by ten-thirty in the morning it is light enough to be able to identify a bird on their feeder, and at two the sun sets again. And I know when the sun is up it is not UP, it is just above the horizon and crawls along almost parallel to the ground, barely clearing the trees, until it sets.

We can't very well complain about light deprivation here: The sun now is coming up at seven, and setting about five.  Nevertheless for someone who wants to study and photograph insects and other invertebrates, this is a severe downtime, since the cold-blooded creatures for the most part have gone into hiatus in some hibernating form--egg, larva, pupa, or, more rarely, adult. In some years in Arkansas I have seen shirt-sleeve weather in December, with a few butterflies and dragonflies still active, but this year we have already had a period of weeks with a solid sheet of ice on the ground, with the temperature at our house coming above freezing only long enough for a three-inch rain, and then going back into freezing temperatures most nights. Days are sometimes gloomy, but sometimes clear and still, with our beautiful winter sunrises, an orange ball rising from a slightly misty row-crop field. That's worth something, I have to admit, especially when it is complemented in the evening by an equal moonrise.

This is our backyard pond iced-over in winter. In summer it is so crowded with aquatic plants you can't see the surface of the water.

Well, I thought I would go out on days when it rose above freezing to see what invertebrates I could find. There are some winter specialists that endure the discomforts in order to avoid the competition of a more crowded season. There is, for instance, a moth that only comes out in the depth of winter. On several occasions in past winters we have been driving at night in light snowfalls and noticed that the snow flakes in the beam of the headlights were not all spiraling down; some were flying up. It was these moths, even in that kind of weather. (We have seen a few out this winter, but not managed to get a photograph.) The idea, as I understand it, is that there are no bats, their most serious aerial predators, around in winter. There also is no suitable food for them, but in fact many moths have no mouth parts and do not feed anyway. They get by on fat stored up in their larval form. It also is not warm, thought to be essential for all cold-blooded creatures, but certain insects find their way around that as well. Bernd Heinrich, my favorite biologist/writer, has demonstrated that many insects are in fact warm-blooded, using shivering and muscle contractions to create an inner temperature well above ambient. The major part of a moth's life is as a caterpillar. At the end of that period, it becomes a pupa, then emerges essentially as flying genitalia. It flies briefly, trots its stuff, mates, lays its eggs, and it has done its work and created a new generation of caterpillars. The winter moth does this in a less complicated world.

We knew about the moths, but what else is out?

There are spiders. I had in the past noticed spiders out, it seemed like, in all kinds of odd parts of the year. One reason I decided to study them seriously this past year is because it seemed like their season was much longer than that of most insects, and that would give my studies less winter down time. So I kept my eyes open, and on days when it warmed up to say the 40s I noticed small wolf spiders running through the leaf litter ahead of me as I walked. They were tiny, and kept moving, and often dived under the leaves, but with a little patience I got a picture of one, good enough to ID it.

It was a spiderling, not an adult, and it was tiny, perhaps 3-4 mm long (the adult is only about 5-6 mm) but I blew the photo way up and could see enough details for the identification. The right-angle long bristles on the legs told me it was in the genus Pardosa; the marking on the abdomen made it P. milvina. Not a surprise, this was the commonest spider around. It was the first spider I identified when I began my studies in March of this year, seeing them running on ahead of me as I walked through my yard, and I had seen them just about every day since then, and my guess is that they will end up being out in every month of the year. Now here is a problem: you see them by the thousands during the year, but almost never carrying prey, to the point where some biologists say they must be very inefficient hunters. Now, some (stingy?) biologist has demonstrated that they can live up to two years without eating. How can that possibly be? There is almost nothing to them to start with. And why do they keep on running, when almost every other invertebrate does a long pause during the coldest months? Mysteries.

Pardosa milvina comes out day and night, but many other wolf spiders come out only at night. I wondered, would a winter night op be feasible? December 28th it warmed up a bit, and an hour or so after dark it was still in the low 40s. I put on my head lamp and went out. There weren't a million eyes shining back at me as in summer, but there were SOME eyes shining back. It was mainly wolf spiders, as I had guessed it would be, but not all. The first spider I saw was a funnel-web spider (recognizable by its striped pattern and long spinnerets), not in a funnel web but out wandering around, doing whatever it is they do in winter. I had seen one earlier scavenging a dead insect, so perhaps that is something they do.

Farther along I found a half grown Nursery Web Spider (Pisaurina mira). This is another spider that is very common in the summer. They make no web, but sit on top of vegetation and wait for something to come close enough to grab. I have no idea what they do during the winter and was very surprised to see it out.

But it was the wolf spiders I saw the most of, perhaps some twenty lights reflected back at me. Most were peering out from under a cave of fallen leaves. When the leaves come down in the autumn they pile up to a depth of two or three inches in our yard, and they are so full of insect eggs and pupae that I don't rake them till spring when next year's growth is underway. That makes it harder to find and approach the wolf spiders, as they don't come very far into the open, and can disappear simply by stepping back.

What I guess most wolf spiders do is sit back in a burrow or a place like these leaves, and lie in wait to ambush whatever comes by. But what does come by in winter? I suppose other spiders. All of the wolf spiders I saw were of this one species. This one popped out far enough for me to identify it.

It was Gladicosa pulchra, one of the commonest wolf spiders, and I was beginning to put together a life history for it. In early spring I only found adults, and I found all of them on tree trunks close to the ground. (I read that at that season they were virtually always on the trees.) Their often elaborate patterns blended perfectly with the moss and bark, and I found this species in all my night ops. Later in summer I began to see them off the trees, running on the ground, and they were out in daytime as well, carrying all their babies on their backs. As summer went on, they became the commonest species to see in the leaf litter. Now it was winter, and they were still common, but now it was all half-grown individuals, the slightly grown up spiders that had been carried on the backs of their mothers in the summer. I had in fact read for this species that they spent the winter as immatures, so that they could become adult early in the spring to mate and lay their eggs, so their young would have a good start in the summer, a time of plenty.

But spiders in winter are not the only creatures around. There are also lots of flies, those tender creatures. I have more than once in past winters seen blue-bottle flies buzzing around actively in temperatures down to 28 or 29 degrees. They were the only invertebrates I had seen active in actually freezing weather. Probably they had some sort of antifreeze in their cells, and used the various ways insects used to maintain operating temperatures. But, I wondered, what are they doing out in winter? Then it occurred to me: blue-bottle flies feed largely on vertebrate carrion, and there is probably more carrion available during harsh and unforgiving winter than at other times.

On the 24th of December I thought I was seeing a blue-bottle out on a cold day, and I took a quick snap of it before it flew away. When I downloaded it I was quite surprised to see it was actually a Tachinid fly.

Tachinids are recognizable by the long spines on their abdomens (they are often called hedgehog flies because of them). They, along with Braconid wasps, are the great population controllers, keeping other insects from burying us under their exponential productivity. Adult Tachinids are a major component of the bees, wasps, butterflies, and other insects that crowd onto flowers in summer to drink nectar, and they are in fact valuable pollinators. But when they are not doing that, they are laying eggs on all manner of invertebrates which their larvae are parasitic on.

So what was this fly doing out in winter? I suppose it could be searching for caterpillars or cocoons that were overwintering, and lay eggs on them that would be ready to hatch in the spring. But then what? Their larvae would quickly finish eating their hosts, and, what? Wait as a pupa until next winter to come out and start another generation? That doesn't seem very sensible. It's another mystery.

There is one more species I want to talk about, that is always out in winter, and that is quite remarkable: The Winter Crane Fly. You could not have a more frail, a less durable appearing creature.

I have blown this picture up so you can see it more clearly, but this exaggerates it greatly. It is only about 5 mm long, which is to say, around a quarter of an inch. It is smaller and slighter than most species of mosquito. In fact, as I'm writing this, I have stepped outside my study and found one on the wall. This gives you a better sense of it:

Now, in bleak midwinter, if it warms up slightly, you may see a little smoke cloud of these tiny creatures whizzing around in a courtship swarm. Here is the best I could manage for a picture of it:

The Winter Crane Fly is called a crane fly, and looks like one, but it actually is only a relative to the crane flies. Here is a typical crane fly:

There are hundreds of kinds (480 species in North America) of these "daddy longlegs" in all sizes from quite large (three inch wingspan) to quite small. They are abundant here in spring and fall. Locally the larger ones are called Skeeter Hawks, under the folk notion that if they look like mosquitoes, but are much larger, they must feed on mosquitoes. Technically they are in the family Tipulidae, and their salient feature is that they have no ocelli. Flies and perhaps most insects have two large compound eyes, and on the forehead between those eyes, three simple eyes, or ocelli, usually arranged in a triangle. See, for example, this wasp:

Here is a detail from the crane fly above, showing its absence (is that a possible construction?) of ocelli:

All this is leading up to the fact that the Winter Crane Fly is in the family Trichoceridae, separate from the true crane flies because it DOES have ocelli. Now, I have known that fact for some time, but the Winter Crane Fly is so tiny that I have never been able to actually see the ocelli. So I got my camera with the 100 mm macro lens, and attached my extension ring to it to give me even closer focus, and got as close as I could to a Winter Crane Fly, and tried to take a picture that would show the ocelli when I blew it up on my computer screen. But the best picture I could get didn't show much of anything.

So I took desperate measures: I collected a specimen.

Herschel Raney and I collected hundreds of specimens of robber flies for vouchers when we were helping to create the state list of robber flies, however both of us would rather do things bloodlessly and take photos instead of specimens. But in this case I wanted to put a Winter Crane Fly under the microscope to look for its ocelli. The best way to kill an insect you want for a specimen is to put it in the freezer. A couple of years ago when I was working on grasshoppers and I needed a specimen to examine under the microscope, I put a grasshopper in a plastic container and put it in the freezer, and after some twenty minutes when I checked, it was dead and already so brittle that with the slightest touch one of its legs came off.

So, I caught a Winter Crane Fly, put it in a plastic container and put it in the freezer. After twenty minutes I took it out and looked at it, and it stood there looking back at me. Hmm. So I put it back in the freezer. Then I went in the house and Cheryl and I had our morning coffee and finished working a crossword puzzle, and I was given a list and drove into town to do some shopping, and by the time I got back two hours had passed since I put the fly in the freezer. I took it out and looked at it, and it stood there and looked back me.

Anyway, it was slowed down enough that I put it under the microscope, and even though it kept walking off and I had to reposition it, finally it stayed still long enough that Cheryl could use her little camera to take a shot of it through the microscope and finally get a rather poor view of one of the ocelli, which I guess would be an ocellus.

If you look at the little brown knob which is the base of the antenna closest to you, to the right of it and touching it you will see a tiny black dot. That's it, the only one of the three ocelli that is in view from that angle. As for the fly itself, I put it outside, and after a few minutes it flew off.

As for the fly's rationale: Coming out in winter, if it stays up on walls of the house where I find it, or up in vegetation, it will avoid the wolf spiders, and there is not much else around to predate it. As to where to lay its eggs, it's larvae feed on rotting fungi or decaying vegetable matter, and that's abundant and easy to find at any time of year. And how does it manage vigorous mating swarms in temperatures not much above freezing? Well, my accidental experiment suggests that it must be running on straight antifreeze (sugar/alcohol in the blood can produce it) backed up by wing-muscle warmup exercises.

Is it a good strategy, coming out in winter? Trichoceridae, the winter crane flies, are found not only here in the north temperate regions but in South America and Australia/New Zealand, suggesting the genus Trichocera has survived over an immense period of time, certainly from before the continents went their separate ways.