Living With Mites

Predatory MItes

Predatory MItes

This above image, Mites On Pupa, by Rogelio Moreno Gill won honorable mention at the 2015 Nikon Small World Photo Competition. Stacking software was used to merge a series of images for improved depth of field. The complex shapes and splashes of color make the image compelling.

Insects are vulnerable to parasites such as mites. Mites are related to spiders with eight legs and chelicera (fangs) that can be used to feed on surface secretions or to puncture the insect cuticle. Some mites feed on insect fluids such as hemolymph. Mite salivary proteases may present insect blood collagulation and viruses may be injected along with saliva. Mites are small enough that they can feed on an insect without killing it, unlike spiders that consume an entire insect at one feeding. However, substantial populations of mites can weaken a host insect and sometimes kill it.

Mites can be problematic for many insect colonies. Mites that infest honey bee colonies can kill larvae and pupae if present in large numbers. They also can spread pathogens such as viruses. Mites are an important factor in the loss of bee colonies over the winter.


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Living With Nitrogen


Helichus striatus
Photo: Jeff Gruber & Nicolas Gompel

It is possible to create a plastron of pure oxygen by placing beetles without plastrons into an atmosphere of pure oxygen. When beetles with oxygen plastron submerge, the plastron shrinks over the first several days, then stabilizes. A plastron created with air does not. Why?

The air of the plastron is in equilibrium with gasses dissolved in water. In a natural air plastron, the insect uses oxygen from the plastron for respiration depleting the oxygen concentration in the plastron. Oxygen dissolved in the water diffuses into the plastron and maintains oxygen concentration in the plastron at a steady state.

In an oxygen-only plastron, oxygen from the plastron will diffuse into the water.  Nitrogen in the water will diffuse into the plastron but oxygen diffusion is more rapid and the plastron shrinks. The insect is also removing oxygen from the plastron. Eventually the plastron reaches a steady state where the concentration of gasses in the plastron balance concentration of gasses in the water.

Plastrons are an interesting adaptation for breathing under water. The plastron must be large enough to support the rate of oxygen consumption of the insect. As plastron size increases, stability decreases. Theoretically, a large enough plastron could support a human underwater. Practically the massive size necessary would reduce mobility.

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Making Bubbles


Line drawing showing a bubble of air collected by the beetle
Image: Hilda Harpster*

The plastron of the long-toed beetle, Helichus striatus, does not form spontaneously when the beetle submerges. The beetle actively creates a plastron in a process that requires at least a half hour and more typically takes a couple of hours.

A beetle without a plastron will climb a stick so that only its mouthparts break the surface of the water. Air is trapped in the space between the mouthparts and the thorax when the beetle submerges. As the mouthparts repeated break the surface and submerge, the bubble expands and pushes backward along the sides of the beetle. Brush-like hairs on the front legs move air from the bubble into the hairs of the plastron on the head and thorax. Movement of the pronotum moves air backward under the elytra.

Eventually all the legs and body regions move to distribute the air in the bubble into the plastron. These behaviors cease once the plastron is complete. How does the beetle sense that its plastron is missing or that its plastron is complete? Only the beetle knows for sure.

*Hilda T. Harpster. An Investigation of the Gaseous Plastron as a Respiratory Mechanism in Helichus striatus Leconte (Dryopidae). Transactions of the American Microscopical Society, Vol. 60, No. 3 (Jul., 1941), pp. 329- 358.

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Groomed For Submersion


Mesothoracic leg showing brush airs used for grooming
Image: Hilda Harpster*

The beetle, Helichus striatus, has hydrofuge hairs that hold the plastron (air bubble) in place. If the plastron is lost, the hydrophobic nature of the hairs dissipate and the hairs become hydrophilic.  The hydrophobic coating must be replaced.

Beetles that lose their plastron will climb on sticks to the surface and begin grooming. The hydrophobic substance appears to come from beetle salivary secretions. The beetle has brush-like hairs on its front legs that are repeatedly drawn through its mouthparts during grooming. The brushes of the prolegs are rubbed repeatedly over the head and prothorax and rubbed together with the middle legs. The middle and hind legs are used to groom the wing and posterior portions of the insect.  During grooming, beetles may climb completely out of the water, spreading their wings and raising their abdomen to allow the beetle surface to completely dry.  After the grooming is complete, the beetles hairs are once more hydrophobic and able to form a plastron.

*Hilda T. Harpster. An Investigation of the Gaseous Plastron as a Respiratory Mechanism in Helichus striatus Leconte (Dryopidae). Transactions of the American Microscopical Society, Vol. 60, No. 3 (Jul., 1941), pp. 329- 358.

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Living With Plastrons


Helichus striatus
Photo: Jeff Gruber & Nicolas Gompel

Helichus striatus is a “long-toed” beetle in the family Dryopidae. Although these beetles are aquatic, they do not swim, but cling to sticks and other floating detritus. They lack physical gills that are present in many groups of aquatic insects. Instead they create a plastron, an air bubble used for breathing.

In 1936, Microscopist, Hilda Harper, investigated the contribution of plastrons to oxygen consumption and survival underwater. The beetle, Helichus striatus, is able to survive over 40 days underwater. The beetle has branched water repelling hairs capable of holding an air bubble in place under its elytra and much of its body when submerged. The spiracles of the beetle open into parts of the plastron.

Harpster measured oxygen content of water with and without beetles and found that beetles were actively removing oxygen from the water. When she placed beetles in oxygen free water, survival was reduced significantly to between 2 and 5 days.

Having established the ability to acquire oxygen from the water, Harpster investigated the role of the plastron. Plastrons can be removed from submerged beetles by gentle brushing. Harpster found that complete removal of the plastron reduced longevity from as many as 40 days to less than 7 days. Partial removal of the plastron had an intermediate effect. Beetles without a plastron had no better survival in oxygenated water than beetles with no oxygen in the water. These experiments provided support for the hypothesis that plastrons were important for underwater respiration.

Hilda T. Harpster. An Investigation of the Gaseous Plastron as a Respiratory Mechanism in Helichus striatus Leconte (Dryopidae). Transactions of the American Microscopical Society, Vol. 60, No. 3 (Jul., 1941), pp. 329- 358.

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Living With Underwater Insects

Riffle Beetle

Riffle Beetle, Elmis aenea
Photo: A. Herrmann

Many insects have larvae that are aquatic but an adult that is terrestrial. Until early in the 20th Century, it was widely assumed that no adult insect could live under water for sustained periods. The French Biologist, F. Brocher, changed these beliefs with his observations of Elmis aenea that were reported in his 1911 paper, Recherches sur la respiration des insectes aquatiques adultes. Brocher observed beetles living underwater for extended periods and noted a layer of air held close to its body by hydrofuge hairs on it cuticle or plastron. Brocher suggested that the plastron was involved in respiration. His observation stimulated further research into plastrons as breathing mechanisms in adult insects.

Elmis aenea is a riffle beetle in the family Elmidae. These beetles are common in fast running water that is well aerated and near lake shores where wave action aerates the water.

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Flickering Bug Lights

Light Trap

Moths and Other Insects Attracted to a Mercury Vapour Light

Question: Why did the moth visit the psychiatrist’s office?
Answer: The porch light was on.

Attraction of insects to porch lights can create a buzz of annoying flying insects and an unsightly pile of dead ones. To reduce the nuisance, homeowners will invest in bug lights, bulbs that are less attractive to insects but produce a ghastly yellow light. Is there a high tech alternative?

LED technology allows light bulbs to be created with novel features. Light intensity, wavelength and flickering can be tightly controlled. Humans have a “critical fusion frequency” (CFF) of around 60 Hz. A light that flickers at a frequency of less than 60 Hz clearly flickers. However, a light that flickers at a rate faster than 60 Hz will appear the same as a light that is constantly on. This affect is caused by the speed that our brain processes light.

Insects have a much higher CFF rate than people. The honey bee CFF is around 240 Hz. If a light is flickering at a rate between 60 Hz and 240 Hz, insects see a flickering light but humans see a steady light. LED lights flickering in this range attract fewer insects.* These experimental results may be the basis of the next generation bug lights.

*A Barroso, I Haifig, V Janei, I da Silva, C Dietrich and AM Costa-Leonardo P.  Effects of flickering light on the attraction of nocturnal insects.  Lighting Res. Technol. 2015; 0: 1–11



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