Beware the water tigers

T. marmoratus Adult

The sunburst diving beetle, Thermonectus marmoratus, is an adept predator. As adults, these Dytiscid beetles are strong swimmers and prey on a variety of aquatic animals by tearing them to shreds with their powerful mandibles. They also spend some time out of water and can fly from one water supply to another. When it is time to reproduce, female diving beetles enter the water and lay eggs on the stems of aquatic plants and macroalgae. When the eggs hatch, the larvae (known commonly as water tigers) enter the water column and begin their rein of terror.

In the lab, these morphologically distinct diving beetle larvae are typically fed tadpoles or mosquito larvae. In the wild, however, they probably eat anything unlucky enough to get too close. When hunting, these beetle larvae either swim around actively or hang, with their tail touching the surface, just below the water line. When they spot a prey animal, they swim over and strike the target with their powerful mandibles (Watch a video of a predation event below). Unlike the adults, larval diving beetles gradually suck the fluids from their prey, resulting in an unfortunately slow demise.

The predatory nature of sunburst diving beetle larvae is highly dependent on their visual system; and boy is it a bizarre one. While the adults have typical arthropod compound eyes, the larvae see the world through stemmata. Stemmata, which are commonly seen in larval insects, are simple lens eyes that rely on superficially similar optical principles to vertebrate eyes. On each side of the head, the larvae have six stemmata as well as a lens-less eye patch (see below). Within each of these eyes there are two distinct retinas, one on top of another. In total, this means that these T. marmoratus larvae have fourteen eyes and twenty-eight distinct retinas!

Front and side views of the head of a T. marmoratus larva. E, eye; EP, eye patch; M, mandible. Adapted from Mandapaka et al., 2006 and Maksimovic et al., 2009.

This larval visual system has a befuddling number of bizarre optical properties. The retinas are sensitive to a broad range of wavelengths, including UV, and the photoreceptor architecture is suggestive of polarization detection. In addition, some of the lenses seem to have novel bifocal and chromatic aberration-correcting properties. Despite the research into all of these strange visual adaptations, the ecological significance of most of the eyes on this animal is completely unknown.

The best understood eyes in the diving beetle larva are E1 and E2. They are forward-looking and primarily used for predation. However, when you look at the main retina in these eyes, you surprisingly find that it is only composed of a thin horizontal band, two photoreceptors tall. Imagine trying to view the world in a thin strip, two pixels high! So, how is the diving beetle larva using these eyes to zero in on prey? Well, it turns out that these sort of strip eyes are not completely novel in nature. Jumping spiders, some copepods, and a pelagic snail all have strip retinas. In order to see the world, they scan their narrow retinas rapidly back and forth, as in the image below. Diving beetles, on the other hand, have absolutely no musculature to move their eyes or retinas. So how do they see?

Look again at the predation video from above. Notice that once the diving beetle larva spots the mosquito larva, it begins bobbing its entire head up and down. The diving beetle larva is scanning the mosquito with the strip retina in its main eyes. As it gets closer, the scanning movement actually becomes more pronounced, since the target takes up more of the field of view. This technique allows the diving beetle larva to accurately hone in on its prey without sacrificing limited head-space for a full retina or eye muscles.

The closer you examine arthropods, the weirder they seem to get. Who would have though that this small aquatic predator would have such a complex and fascinating visual system? In order to discover the most exciting aspects of living things, you need to look. That’s where science starts; with someone peering through the confounding subterfuge of nature, hoping to widen our glimpse of the gear-works within.

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The research discussed in this post is being carried out at Buschbeck lab at the U. of Cincinnati.

References:

• Mandapaka K, Morgan RC, & Buschbeck EK (2006). Twenty-eight retinas but only twelve eyes: an anatomical analysis of the larval visual system of the diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae). The Journal of comparative neurology, 497 (2), 166-81 PMID: 16705677
• Buschbeck EK, Sbita SJ, & Morgan RC (2007). Scanning behavior by larvae of the predacious diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae) enlarges visual field prior to prey capture. Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology, 193 (9), 973-82 PMID: 17639412