The most dangerous thing to a nocturnal insect isn’t flying into a tree, it’s bats. Many bats (most? Batologists, give me an estimate please!) are insectivorous, using their ultrasonic calls (above 20 kHz) to detect anything that flies. In response, there has been a sort of coevolution going on between nocturnal insects and insectivorous bats, with the insects developing hearing systems capable of perceiving bat calls; members of the Coleoptera, Dictyoptera, Neuroptera, Orthoptera and Lepidoptera have all convergently evolved these systems (Fullard & Yack, 1993). It’s the latter that we’re interested in in this post: the moths.
Moths are particularly rich in hearing systems, with both head appendages and more traditional ear-like vibration receptors (tympani) used to detect sound (see possible locations above, modified from Hoy & Robert (1996)). Of course, it’s family-dependent, as these all evolved independently. The tympani are found on various locations on the abdomen, and are tuned to detecting the frequencies used by co-inhabiting bats (Roeder & Treat, 1970). In fact, take the moths or bats out of the habitat, and the tympani become less receptive to ultrasound (Fullard, 1988).
The way a tympanum works is simple. There are three parts to it: the tympanum itself, the tracheal sac, and the tracheal organ. The tympanum is a modified cuticle that is extremely thin, making it responsive to vibrations. But reacting to vibrations is one thing. For there to be a functional response, the inner side of the tympanum needs to have an accoustic impedance, i.e. a density (it’s more complex than that, but this ain’t a physics blog and I’m not bothered to bust LaTeX out) matched to the external environment. This is the role of the tracheal sac, which is just a pocket of air (the same principle applies to the mammalian ear, btw).
With this set-up of a flexible membrane and an air sac, what happens is that a sound wave can cause pressure changes in the air of the tracheal sac. This is nothing more than a mechanical signal (like the vibrating bones in our ear). The final step is conducted by the tympanal organ. It’s made up of scolopidia (fancy name for chordotonal sensillae, or small sensory hairs) pointing towards the tympanum (with some exceptions, e.g. in geometrids). See Field & Matheson (1998) for all the details you would want on them. Each scolopidium is composed of a cell with a cap. An extra cell links the tympanum to the cap. So when the tympanum vibrates, the cap can sense that vibration and transmit it downwards to the cell.
At the base of the scolopidial cell, a cilium is found. On vibration, the cilium’s permeability changes, and this generates an action potential (French, 1998), sending nervous impulses to bipolar neurons (termed A1 and A2), which then link up to the intersegmental auditory interneurons, which then further link up to the medial ventral association center, a mechanosensory processing center (Merritt & Whitington, 1995) – which should give you a clue at why it’s so easy to evolve hearing systems independently, it’s simply a co-option of a preexisting system for mechanoreception.
That was a generalised look at tympanal systems, and of course exceptions and modifications exist. Moths have the simplest systems around (cicadas and crickets are vastly more complex). For example, hawkmoths have a grand total of one (1) scolopidium, while most insects have 50-100 (cicadas have 1000+).
Anyway, I hope that’s enough of an introduction for you to be able to understand at least the abstract of this new paper I was reading today:
ter Hofstede HM, Goerlitz HR, Montealegre-Z F, Robert D & Holderied MW. 2011. Tympanal mechanics and neural responses in the ears of a noctuid moth. Naturwissenschaften 98, 1057-1061.
What they did was test how the noctuid moth tympanum reacts to artificially-produced (and thus precisely-measured) sound waves. Spoiler: their key result is that what determines hearing isn’t the speed at which the tympanum vibrates, but how much it gets moved by the vibration. Also, they found that there was no difference in the neurological responses of intact moths and specially-prepared moths, so rigorous lab studies, which often involve a bit of dissection and other organismal stresses, do provide realistic results. That’s a big relief, as it means that what we study in the lab can be safely assumed to also work the same way in the wild. Makes it easier to plan more experiments.
Anyway, if you’re interested in more details of moth hearing, read that paper. The background info in this post should be enough to understand it; if not, don’t hesitate to ask for clarifications! If there is interest in the subject, but not as detailed as this paper, I might add a post about this in the pile of things to write, if requested.
Field LH & Matheson T. 1998. Chordotonal Organs of Insects. Advances in Insect Physiology 27, 57-228.
French AS. 1988. Transduction Mechanisms of Mechanosensilla. Annual Review of Entomology 33, 39-58.
Fullard JH. 1988. The tuning of moth ears. Cellular and Molecular Life Sciences 44, 423-428.
Fullard JH & Yack JE. 1993. The evolutionary biology of insect hearing. TrEE 8, 248-252.
Hoy RR & Robert D. 1996. Tympanal Hearing in Insects. Annual Review of Entomology 41, 433-450.
Merritt DJ & Whitington PM. 1995. Central projections of sensory neurons in the Drosophila embryo correlate with sensory modality, soma position, and proneural gene function. The Journal of Neuroscience 15, 1755-1767.
Roeder KD & Treat AE. 1970. An acoustic sense in some hawkmoths (Choerocampinae). Journal of Insect Physiology 16, 1069-1086.