Invertebrate Behavior is Fascinating

I spent the better part of a couple hours today watching a stinkbug (Halyomorpha halys). It’s pretty clear that its behavior is programmed pretty rigidly, in a manner that must have worked very well in a world without human-designed objects. This approach is failing this particular bug today.

I did not witness the beginning of the bug’s travels, but the present situation is that the bug is crawling back and forth along the top edge of an interior screen on my screened porch. A round trip (it’s a distance of about 1 m) takes around 5 min 45 sec (I measured three round trips and averaged – longest was about 6 min 10 sec and included an especially long [more than 1 min] turn around at one end). 

The bug crawls to the left until it hits the corner. It then turns downward for about 5 cm. Then it turns around, climbs back to the top and proceeds over to the right side of the screen. Then, once again, it goes down for about 5 cm until it reverses its direction. 

It seems to be executing a very simple program that consists of two components:

  • Crawl.
  • Crawl toward the dark.

Once the bug found itself on the screen, crawling toward the dark would cause the bug to crawl upwards; the major source of light is the great outdoors but it’s overcast today and the porch is shaded by trees. I haven’t measured, but it’s likely that the light energy coming in through the screen is pretty even from side to side and even from top to bottom. However, on the interior side the ceiling is darker than the floor, so from the bug’s perspective upwards is toward a darker region than downwards.

Once the bug hits the top edge, it is well shielded from any light from above. It crawls in either direction. When it hits the corner and then crawls downward, it finds itself entering a lighter area, so it turns around. And so on…

I first noticed the bug around 11:00 AM, and it’s still going now at about 1:40 PM. That means it has made at least 25 round trips, and this had been occurring  before I noticed. If it persists for another hour or two I’ll rescue it and take it outside. [Edit – 1:55 and it’s gone.  2:00 PM – I see that it has made its way to another screen, about 0.5 m from the top and near an edge. It’s now crawling up along that edge… and it has reached to top and is moving across.]

The program probably served this bug’s ancestors well in the pre-engineered world of irregular surfaces. Simple if-then commands executed by disparate ganglia got those earlier bugs around effectively, and because those ganglia are not unified into a central nervous system there’s no reflection, or in this case, boredom. Execute the program and survival will follow. That is, until perfectly horizontal surfaces and right angles appeared.  I don’t think stinkbugs experience existential crises.


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