Finding the Way Home

Cockroach

Female German Cockroach with Ootheca (egg case) protruding from her abdomen

Insects that make nests have a well developed navigation system that allows them to forage outside the nest and return. Hymenoptera make extensive use of visual cues and use a series of landmarks to navigate back to their nests. German cockroaches do not have nests but they aggregate in a harborage with a defined location. German cockroaches forage outside of the harborage, but return to the same harborage. How do they find the way home?

Rivault and Durier* studied the behavior of cockroaches in an arena where visual and olfactory cues could be manipulated. Moving either the olfactory or the visual cues increased the length of the path a cockroach traveled to return to the harborage. The return path was shortest when both olfactory and visual cues were presented in agreement. When cockroaches encountered a visual cue without the olfactory cues, they investigated the area around the visual cue before moving on. If cockroaches encountered olfactory cues without the visual cues, they also investigated the area before moving on.

This behavior may be adaptive to those environments inhabited by cockroaches. Under high light intensity, visual cues are readily apparent and allow a cockroach to reach a harborage by a direct path. Where low light intensity eliminates visual information, a cockroach must rely on other modalities such as olfaction.

*Colette Rivault & Virginie Durier. (2004)  Homing in German Cockroaches, Blattella germanica (L.) (Insecta:Dictyoptera): Multi-Channelled Orientation Cues.  Ethology 110, 761—777.

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Discriminating Sounds

Katydid Knee

Sound Detector (Tympanum) On the Front Leg of a Katydid

Insect hearing organs vary in their ability to discriminate among sounds. Insects that need hearing to do one thing well, such as warning of an approaching bat, can have an auditory system that only responds to sounds made by bats. Some sphingid moths have a single auditory neuron in each tympanum that triggers evasive flight activity.

Using sound to identify, locate and find mates requires more sophisticated auditory processing systems. The auditory organs of cicadas can contain a couple thousand sensory neurons. The Johnston’s organs of male mosquitoes that are capable of sophisticated tone matching can contain 15,000 neurons. In comparison, the human ear contains around 16,000 sensory hair cells in the cochlea.

Martin C. Gopfert and R. Matthias Hennig. Hearing in Insects. Annu. Rev. Entomol. 2016. 61:257–76.

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Zika Transmission in Texas

Mosquito Bite

Mosquito Bite

The Texas Department of State Health Services reported on November 28, 2016 the first documented case of Zika transmission in Texas from Cameron County along the Mexican border. A woman tested positive for Zika virus in urine but not blood indicating that she was exposed but no longer infective. It is unknown if Zika was contracted through mosquito bite or through one of the other transmission pathways. Zika transmission has been reported for communities across the border in Mexico. The question has always been “When will Zika be transmitted in Texas” not “Will it or will it not”.

Texas officials are intensively trapping mosquitoes in an effort to identify local mosquito populations containing Zika. They are also collecting voluntary urine samples to identify the extent of the infection in the local population. The CDC has been notified, but as of yesterday, have not issued travel warnings for Texas.

Texas is recommending Zika testing for all pregnant women. However, Texas has challenges including one of the highest uninsured populations in the US due to the refusal of Texas politicians to expand Medicaid coverage under the ACA.  Comprehensive coverage of women’s health care is not in place because of political and religious objections. Symptoms of Zika are often mild and people who lack medical coverage are not likely to pay out of pocket for a doctor visit or testing.  This creates a climate where new infections and their spread may be ignored.  A culture that views health issues as an individual responsibility clashes with volumes of medical and scientific data on the benefits of public health programs.

Lack of attention to public health can have bad consequences. The West Nile outbreak of 2012 was especially bad with over 200 West Nile associated deaths. Many of the deaths were in Texas. Weather conditions are correlated with the spike in disease and deaths. However, the response and lack of attention to mosquito control in Texas have been questioned. How effective will the Texas response to Zika be? At a national level, we hope that a vaccine will soon be available to protect the population from the worst effects. In the meantime, it is important for all to do what we can to slow the spread into the US.

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Vive La Différence

Mosquito

Mosquito

If two sounds of different frequencies are played together, they can produce a “difference tone” or a sound that has a frequency that is determined by subtracting the frequency of the lower tone from the higher tone. For example, two adjacent strings on a string instrument are separated by a perfect fifth are played together.  In addition to the tone of each individual string, an additional tone that is an octave below the lower tone (or difference tone), is also produced..

Difference tones are used for mate identification and finding by the Culex mosquitoes. A female is larger and beats her wings more slowly than the smaller male. When a male and female of the same species approach the antennae vibrate in response to both the female wingbeat frequency and the male wingbeat frequency. Together, the vibrations create another lower frequency vibration of the antennae that is the difference tone. Although the male and female wingbeats create a high frequency harmonic, the harmonic is above the response range of the antenna. However, the difference tone is in the range of detectable frequencies and it is the difference tone that is important to mate recognition.

Ben Warren, Gabriella Gibson and Ian J. Russell. Sex Recognition through Midflight Mating Duets in Culex Mosquitoes Is Mediated by Acoustic Distortion. Current Biology 19, 485–491, March 24, 2009.
DOI 10.1016/j.cub.2009.01.059

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The Mechanics of Hearing

Cartoon of Mechanically Activated Nerve Channel

Cartoon of Mechanically Activated Nerve Channel*

Flies such as Drosophila hear with their antennae. The base of the antennae, the Johnston’s organ vibrates in response to vibrations in the air (sound). The vibrations mechanically move hairs that are innervated by sound detectors. How is the vibration in hairs and neurons converted into nerve signals?

Nerve signals are generated by ions such as sodium and potassium moving through channels in the nerve membrane. Channels are made from proteins that span the membrane from inside to outside. Ions can cross the membrane through a pore in the center of a protein channel. The pore in the channel has a protein that acts as a gate to opens and close the pore. (See Cartoon) In mechanically activated channels, the protein gate is attached to the actin cytoskeleton of the nerve cell. Deforming the nerve cell causes the actin cytoskeleton to move, which moves the gates and opens pores.

Mechanical coupling acts rapidly. A nerve impulse may be generated in less than a nanosecond after the nerve is deformed. This is much faster than any known chemical activation. The inability of chemical coupling to act rapidly enough led scientists to search for other mechanisms that couple vibration to nerve impulses. The mechanism identified in Drosophila is similar to the mechanisms found in human ear hairs. Thus, Drosophila may be a good model to explore hearing with application to humans.

*Susanne Bechstedt, Jonathon Howard. Hearing Mechanics: A Fly in Your Ear, Current Biology, Volume 18, Issue 18, 23 September 2008, Pages R869-R870.
http://dx.doi.org/10.1016/j.cub.2008.07.069

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Centipede Art

This foreboding image of the fangs of the centipede, Lithobius erythrocephalus, won 13th place at the 2016 Nikon Small World Photomicrography contest. The use of digital stacking keeps the details of the fangs in focus as well as the mouth cavity below. The opening of the mouth has spines that can be used to grasp prey. The mouthparts in the background are used to bite chunks that can be swallowed. Many of the hairs contain receptors responsive to touch, odors or taste.

The fangs of centipedes are modified legs that are heavily sclerotized and sharp at the tips. At the base is a venom gland. The venom can quickly immobilize prey to make handling easier. For people bit by a centipede, the venom can cause intense “bee-sting-like” pain.

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Living With Bat Detectors

Sound Organs

Sound Organs (Pilfers (p)) of A Hawkmoth
Image: L.T. Wasserthal

Hawkmoths fly at night and are tasty prey for bats. Some hawkmoths have evolved the ability to detect the ultrasonic sounds emitted by bat sonar. The Death Head Hawkmoth and some other moths in the Choerocampina, use a sound detection organ that has evolved from a modified labial palp and labral pilifer (a hair covered remnant of the vestigial mandibles). The labral pilifer contains the sensory nerves that send signals to the brain. The palp is modified to vibrate in response to sound. The palp has a sound sensitive segment that is enlarged and contains an air sac. The cuticle covering the air sac is thin and devoid of scales.  The thin membrane backed by an air sac forms a tympanum that vibrates in response to select sound frequencies.

The image on the left shows the right palp in its natural position where it contacts the pilifer. The left palp was moved to reveal the pilifer. When the sound organ on the palp vibrates, it contacts the pilifer.  The movement causes the pilifer to send a signal to the brain.  This unusual hearing organ is only found in a few species of hawkmoth.

Many species of insects can eavesdrop on bats and do so by a variety of sound detectors that have evolved independently.  The number of bat detector types is testament to the toll that bats take on flying insects and the advantages to insects of an early warning system.

Gopfert MC,Wasserthal LT. 1999. Hearing with the mouthparts: behavioral responses and the structural basis of ultrasound perception in Aherontiine hawkmoths. J. Exp. Biol. 202:909–18

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