Monday, November 16, 2020

Netwing beetles: Beautiful and toxic to predators

With over 350,000 described species, the beetles are insects with many different attributes. The scientific name for the beetles, Coleoptera, is based on the Greek word “koleopteros,” meaning sheathed wing. Beetles in the Adirondack region show that this sheath can take a variety of forms.

The outer pair of wings located on a beetle is known as the elytra. For the ironclad beetles, the elytra are so hard that some of these specimens can resist being run over by cars. Ironclad beetles native to New York tend to be small, brown denizens of rotting trees. The drab hardness of ironclad beetles contrasts sharply with the flexible, flashy wings found on net-winged beetles. There are several species of net-winged beetles in New York, including the banded net-winged beetle Calopteron discrepans Newman (Coleoptera: Lycidae), the reticulated net-winged beetle Calopteron reticulatum Fabricius (Coleoptera, Lycidae), and the end band net-winged beetle Calopteron terminale Say (Coleoptera, Lycidae), which is shown in the picture. The transverse depression across the wings, combined with the equal heights of its wing ridges, are the diagnostic features of the end band net-winged beetle.

Net-winged beetles are roughly two centimeters long as an adult. The name of the genus, Calopteron, means “beautiful wing” in Greek. As adults, they can be found along the banks of the Boquet River, and in woodlands. They are active at least until September, and can be seen visiting flowering plants. Even when pitted against the sharp yellows piercing whites of wildflower petals, net-winged beetles stand out. How can such a gaudy insect avoid predators?

The wings of these beetles produce toxins called pyrazines. The chemical is only emitted when the ridges on the wing are broken, suggesting that the chemical is defensive in nature. These foul-smelling chemicals give the insect a distinctly unpleasant taste when predators break the wings as they eat their prey. Entomologists argue that the bright coloration helps the beetle advertise its unpalatability; predators recognize the flashy colors, and remember not to eat another individual that has this appearance. Net-winged beetles have been rejected by wolf spiders, orb weaver spiders, and thrushes, which lends credence to the idea that predators learn to dismiss them. Aposematic coloration is the label given to this strategy. It keeps the beetle alive long enough to feed on plant juices, slime molds, or other insects, and to mate.

Larvae

Little is known regarding the ovipositional strategies of this insect. Does it lay eggs in the soil, on plants, or in some other substrate? Regardless of the answer, the larvae are a bit more understood. With overlapping segments of chitinous armor on their exoskeleton, the juvenile beetles crawl about under the bark of trees, serving the ecological role of a predator. When acting as predators, they consume mostly small arthropods. Furthermore, the larvae can consume rotting wood. This ecosystem service aids in the decomposition of trees, which helps facilitate the carbon cycle. When the insects are ready to become adults, they congregate en masse on the surface of trees. Next, they pupate in their huddle. Why should these insects bother with the task of arranging themselves in such bundles?

Larvae molt multiple times. Like the adults, the larvae have aposematic coloration. When they are ready to pupate, net-winged beetle larvae retain the outer layer of their exoskeleton. By retaining the coloration of the flamboyant, brightly-colored larvae, it is believed that the pupae benefit from the protection conferred by aposematism.

Toxic to predators

Net-winged beetles do not rely exclusively on the strategy of using their predators’ intelligence against them. Reflex bleeding is also demonstrated by this beetle. When threatened, they ooze toxins from the gaps between their leg joints. This unpalatable liquid can further deter predators.

Other insects employ the strategy of aposematism. With respect to the net-winged beetle mimicry complex, there are several moths and beetles that share the same coloration and general body shape. For example, the Pyromorpha and Lycomorpha moths have orange wings with black bands. The moths can be distinguished from beetles because the moths lack chewing mouthparts, and their antennae lack the long, thick, serrated prongs found on the beetle. Both the Lycomorpha moth and the net-winged beetle are poisonous, so the relationship is considered an example of Mullerian mimicry, where both the model and the mimic are unpalatable to prey. The plan is not foolproof; a beetle Elytroleptus divisus LeConte (Coleoptera: Cerambycidae) mimics the appearance of net-winged beetles, and yet preys upon them without getting sick from the toxins.

Net-winged beetles may not be the most durable insects, or the fastest fliers. Nonetheless, these specimens have useful lessons to teach us about using the perceived advantages of foes against them. Net-winged beetles have developed such formidable defenses that they can demonstrate beauty in a hostile environment without fear, a trait that other species may find enviable.

An end band net-winged beetle Calopteron terminale Say (Coleoptera: Lycidae), found near the Boquet River, during September 2020.

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Shane Foye is an entomologist volunteering with the Cornell Cooperative Extension of Essex County Agriculture Program. Learn more about Shane and the on-farm projects he's working on at the CCE Essex Agriculture page: http://essex.cce.cornell.edu/agriculture/biocontrol-projects




One Response

  1. Excellent informative article! Thank you Shane!

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