The Cricket Sensory System: What’s a cricket to do?

Mechanical Host Defenses:

~Chewing mouthparts-biting an attacker. Not useful against O .ochracea

~Limb autotomy- ‘quick release’ legs. Not useful against O. ochracea

~Vomiting-once caught crickets vomit in an effort to repulse predator. Not useful against O. ochracea

Immune Defence:

A typical immune response involves encapsulation, which is surrounding a foreign body with hemocyctes. Hemocytes continue to accrue, eventually killing the intruder. However, when parasitoids infect their regular host this type of reaction rarely occurs (in G. texensis the hemocytes released are incorporated into a funnel, allowing the parasitoid to breath through the cricket’s tracheal system[i].

Ways that parasitoids/parasites overcome the hosts’ immune response:

~Avoidance- as in ectoparasites; immune incapable life history stages; hemolymph avoidance

~Evasion-parasite is ‘cloaked’ in host mimicking proteins

~Destruction-feeding to weaken host or directly damaging hemocytes

~Subversion-the ability to develop despite a host response

To investigate whether G. texensis can detect the presence of O. ochracea, it is necessary to understand the cricket sensory system.

Mechanoreception has three stages:

~mechanical event is coupled to the receptor cell membrane by mechanical structures

~event is transduced into a receptor current at the cell membrane

~information encoded into an action potential for transmission to CNS

Mechanoreception allows insects to detect their external and internal mechanical environments[ii]. The most basic mechanoreceptors are trichoid sensilla.   These are setae that have a sensory neuron located at the base.  Dendrites of the neuron generate nerve impulses whenever movement is detected (setae moves).

Figure5: Trichoid Sensilla

Image courtesy of Dr. Meyer of NC State University, Entomology Department

In crickets the cerci have 5 types of sensilla, filiform hairs, dome shaped campaniform, clavate hairs, long slender bristles and short bristles[iii]. Cercal hairs are sensitive to air currents which may aid in the detection of the parasitoid fly[iv].

The direction of the approaching animal is indicated by which hairs are deflected and

the time-varying pattern of the air movement gives information about the type of animal approaching.

Another type of mechanical receptor is the campaniformii. These bell-shaped sensilla are found on the cuticle in compact groups near joints. These sensilla are only excited when the hair socket that they surround is deflected and their response is phasic[v]. Stress moves the bell inward compressing the tip of the dendrite, letting the insect know that a pressure has been detected on the cuticle.

Figure 6 : Male cricket with cerci

Photo courtesy of www.fcps.k12.va.us/.../fldcrckt.gif

If these receptors are sensitive enough, it is possible that G. texensis uses them to detect the presence of an adult O. ochracea in search of a place to release her planidia, or to detect the planidia itself. There is one recorded incident of a gravid O. ochracea landing on a calling G. texensis that initiated intense grooming and was later found to be free of parasites[vi]. This suggests that the cricket was able to detect when the fly landed on it and associated that stimulus with the initiation of a grooming response (anti-parasitism).

Figure7: Campaniform Sensilla

Image courtesy of Dr. Meyer of

NC State University

Chordotonal receptors serve several functions including hearing and joint movement and are located beneath the integument. Subgenual organs (comprised of chordotonal receptors) located in the legs of many insects are sensitive to substrate vibrationsii. Tympanal organs, for example, lie beneath the tympanum where they respond to sound vibrations. The cricket ‘ear’ or, tympanal membrane, is a thinned region of exoskeleton located on either side of a cricket’s foretibia.

In the cricket, sound waves contact both the outer and inner surface of the tympanum. The tympanum then vibrates in response to the pressure difference between the two sides providing information about the direction of the sound[vii]. This aids in both mate location (for females orienting towards a male’s call) and predator avoidance. It is unlikely that crickets can acoustically avoid O. ochracea because the frequency of the fly’s wing beat (~200Hz)[viii] is much higher than the cricket’s peak ‘hearing’ range of 4-5 kHz[ix]. Therefore, the fly’s wing-beat frequency is too low to be detected by the cricket’s far-field hearing organ (tympanum); it is more likely that the cricket be able to detect the fly’s near-field sound.

Figure 8: Chordotonal Receptor

Image courtesy of Dr. Meyer of NC State University, Entomology Department

Filiform hairs (as earlier mentioned) do not only respond to tactile stimulation (physical displacement) but also to changes in the particulate composition of the surrounding air (near-field sound)v.  The filiform hairs on the cerci of the common house cricket, Acheta domesticus, respond to frequencies between 50 and 3000Hz with the greatest sensitivity between 50 and 400Hz. By combining filiform hairs with campaniform sensilla the speed of air currents may be measured over a wider range than each of the two types of sensilla is capable of detecting alone; this is useful in determining an unthreatening from a threatening air disturbancev. Furthermore, A. domesticus can detect the wing beat of a flying wasp (150Hz) at distances of up to 20cm with its cercal hairs[x]. It is therefore likely that G. texensis’ cercal sensilla allow it to detect the terrestrial approach, and possibly the aerial approach (wing-beat) of O. ochracea. 

Crickets have both simple and compound eyes with which they may be able to detect the approach of O. ochracea. In general, insects have poor resolution because compound eyes have small lenses and the blur circle resulting from diffraction is inversely proportional to aperture diameterii. Simple eyes are sensitive to changes in light while compound eyes are sensitive to image motion, because of this, compound eyes are used for navigation, predator avoidance and mate findingii. Ocelli, or simple eyes, are usually arranged in the shape of a triangle on top of the head. They arecovered by transparent cuticle and overlay transparent epidermal cells allowing light to pass straight through to the retina. Compound eyes consists ofrepeated ommatidia units. Each ommatidia is isolated, having its own group of retinula cell (each with unique spectral sensitivity), and its own ring of pigment cells.

Figure 9: Compound and simple eye of arthropods.

 Images courtesy of cas.bellarmine.edu/.../Bio%20Pix.htm

It has yet to be investigated if and how G. texensis may be using its vision to avoid parasitism by O. ochracea.  Though crickets are nocturnal, they emerge at dusk when O. ochracea is most active[xi], so it is possible that G. texensis is visually avoiding parasitism, even if only for this short period. However, previous studies suggest that visual cues are not important in detecting and avoiding predation in A. domesticusx. They found that covering the eyes in A. domesticus did not prevent the animal from responding defensively to the approach of a predatory wasp.

I will not be investigating the potential use of chemosensory cues in the detection of O. ochracea, but I will here mention that insect olfactory receptors are thin-walled pegs, cones, or plates. These receptors have numerous pores through which airborne molecules diffuse.

Figure10: Chemoreceptor

Image courtesy of Dr. Meyer of NC State University, Entomology Department

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