Sensory Systems/Butterflies

Johnston's Organs (Antennae in Bees and Butterflies)

The sense of balance of butterflies sits at the base of the antennae.

Butterflies and moth keep their balance with Johnston's organ: this is an organ at the base of a butterfly's antennae, and is responsible for maintaining the butterfly's sense of balance and orientation, especially during flight.

Introduction

The perception of sound for some insects is important for mating behavior, e.g. Drosophila . The ability of hearing in Insecta and Crustacea is given by chordotonal organs: mechanoreceptors, which respond to mechanical deformation . These chordotonal organs are widely distributed throughout the insect’s body and differ in their function: proprioceptors are sensitive to forces generated by the insect itself and exteroreceptors to external forces. These receptors allow detection of sound via the vibrations of particles when sound is transmitted though a medium such as air or water. Far-field sounds refer to the phenomenon when air particles transmit the vibration as a pressure change over a long distance from the source. Near-field sounds refer to sound close to the source, where the velocity of the particles can move lightweight structures. Some insects have visible hearing organs such as the ears of noctuoid moths, whereas other insects lack a visible auditory organ, but are still able to register sound. In these insects the "Johnston's Organ" plays an important role for hearing.

Johnston's organ

The Johnston’s Organ (JO) is a chordotonal organ present in most insects. Christopher Johnston was the first who described this organ in mosquitoes, thus the name Johnston’s Organs . Quarterly Journal of Microscopical Science. 1855, Vols. s1-3, 10, pp. 97-102.. This organ is located at the stem of the insect’s antenna. It has developed the highest degree of complexity in the Diptera (two-wings), for which hearing is of particular importance . The JO consists of organized base sensory units called scolopidia (SP). The number of scolopidia varies among the different animals. JO has various mechanosensory functions, such as detection of touch, gravity, wind and sound, for example in honeybees JO (≈ 300 SPs) is responsible to detect sound coming from another “dancing” honeybee . In male mosquitoes (≈ 7000 SPs) JO is used to detect and locate female flight sound for mating behavior . . The antenna of these insects is specialized to capture near-field sound. It acts as a physical mechanotransducer.

Anatomy of the Johnston’s Organ

A typical insect antenna has three basic segments: the scape (base), the pedicel (stem) and the flagellum . Some insects have a bristle at the third segment called an arista. Figure 1 shows the Drosophila antenna. For the Drosophila the antenna segment a3 fits loosely into the sockets on segment a2 and can rotate when sound energy is absorbed . This leads to stretching or compression of JO neurons of the scolopidia. In Diptera the JO scolopidia are located in the second antennal segment a2 the pedicel (Yack, 2004). JO is not only associated with sound perception (exteroreceptor), it can also function as a proprioceptors giving information on the orientation and position of the flagellum relative to the pedicel .

Figure 1: Left: Frontal view of the Drosophila antenna. The scolopidia in the second segment (a2, pedicel) with their neurons are illustrated. Sound energy absorption leads to vibration of the arista and rotation of the third segment a3. The rotation leads to deformation of the scolopidia, leading to activation or deactivation. Right: The antenna located on the head of the Drosophila is shown. (adapted from ).

Structure of a Scolopedia

A scolopidia is the base sensory unit of the JO. A scolopidia comprises four cell types : (1) one or more bipolar sensory cell neurons, each with a distal dendrite; (2) a scolopale cell enveloping the dendrite; (3) one or more attachment cells associated with the distal region of the scolopale cell; (4) one or more glial cells surrounding the proximal region of the sensory neuron cell body. The scolopale cell surrounds the sensory dendrite (cilium) and forms with this the scolopale lumen / receptor lymph cavity. The scolopale lumen is tightly sealed. The cavity is filled with a lymph, which is thought to have high potassium content and low sodium content, thus closely resembling the endolymph in the cochlea of mammals. Scolopidia are classified according different criteria. The cap cell produces an extracellular cap, which envelopes the cilia tips and connects them to the third antennal segment a3 .

Type 1 and Type 2 scolopidia differ by the type of ciliary segment in the sensory cell. In Type 1 the cilium is of uniform diameter, except for a distal dilation at around 2/3 along its length. The cilium inserts into a cap rather than into a tube. In Type 2 the ciliary segment has an increasing diameter into a distal dilation, which can be densely packed with microtubules. The distal part ends in a tube. Mononematic and amphinematic scolopidia differ by the extracellular structure associated with the scolopale cell and the dendritic cilium. Mononematic scolopidia have the dendritic tip inserted into a cap shape which is an electron dense structure. In amphinematic scolopidia the tip is enveloped by an electron-dense tube. Monodynal and Heterodynal scolopidia are distinguished in their number of sensory neurons. Monodynal scolopidia have a single sensory cell and heterodynal ones have more than one.

JO studied in the fruit fly (Drosophila melanogaster)

The JO in Drosophila consists of an array of approximately 277 scolopidia located between the a2/a3 joint and the a2 cuticle (a type of an outer tissue layer) . The scolopidia in Drosophila are mononematic . Most are heterodynal and contain two or three neurons, thus the JO comprises around 480 neurons. It is the largest mechanosensory organ of the fruit fly . Perception by JO of male Drosophila courtship songs (produced by their wings) makes females reduce locomotion and males to chase each other forming courtship chains . JO is not only important to perceive sound, but also to gravity and wind sensing. Using GAL4 enhancer trap lines in the JO showed that JO neurons of flies can be categorized anatomically into five subgroups, A-E . Each has a different target area of the antennal mechanosensory and motor centre (AMMC) in the brain (see Figure 2). Kamikouchi et al. showed that the different subgroups are specialized to distinct types of antennal movement . Different groups are used for sound and gravity response.

Neural activities in the JO

To study JO neurons activities it is possible to observe intracellular calcium signals in the neurons caused by antenna movement . Furthermore flies should be immobilized (e.g. by mounting on a coverslip and immobilizing the second antennal segment to prevent muscle-caused movements). The antenna can be actuated mechanically using an electrostatic force. The antenna receiver vibrates when sound energy is absorbed and deflects backwards and forwards when the Drosophila walks. Deflecting and vibrating the antenna yields different activity patterns in the JO neurons: deflecting the receiver backwards with a constant force gives negative signals in the anterior region and positive ones in the posterior region of the JO. Forward deflection produces the opposite behavior. Courtship songs (pulse song with a dominant frequency of ≈ 200Hz) evoke broadly distributed signals. The opposite patterns for the forward and backward deflection reflect the opposing arrangements of the JO neurons. Their dendrites connect to anatomically distinct sides of the pedicel: the anterior and posterior sides of the receiver. Deflecting the receiver forwards stretches the JO neurons in the anterior region and compresses neurons in the posterior one. From this is can be concluded that JO neurons are activated (i.e. depolarized) by stretch and deactivated (i.e. hyperpolarized) by compression.

Different JO neurons

A JO neuron usually targets only one zone of the AMMC, and neurons targeting the same zone are located in characteristic spatial regions within JO . Similar projecting neurons are organized into concentric rings or paired clusters (see Figure 2A).

Vibration sensitive neurons for sound perception

A and B neurons (AB) were activated maximally by receiver vibration between 19 Hz and 952 Hz. This response was frequency dependent. Subgroup B showed larger response to low-frequency vibrations. Thus subgroup A is responsible for the high-frequency responses.

Deflection sensitive neurons for gravity and wind perception

C and E showed maximal activity for static receiver deflection. Thus these neurons provide information about the direction of a force. They have a larger displacement threshold of the arista than the neurons of AB . Nevertheless CE neurons can respond to small displacement of the arista (e.g. gravitational force): gravity displaces the arista-tip by 1 µm (see S1 of ). They also respond to larger displacement caused by air-flow (e.g. wind) . Zone C and E neurons showed distinct sensitivity to air flow direction, which causes deflection of the arista in different directions. Air flow applied to the front of the head resulted in strong activation in zone E and little activation in zone C. Air flow applied from the rear showed the opposite result. Air flow applied to the side of the head yielded in zone C in ipsilaterally activation and in zone E in contralaterally one. The different activation allows the Drosophila to sense from which direction the wind comes. It is not known whether the same subgroups-CE neurons mediate wind and gravity detection or if there are more sensitive CE neurons for gravity detection and less sensitive CE neurons for wind detection . A proof that wild-type Drosophila melanogaster can perceive gravity is that the flies tend to fly upwards against the force vector of gravitation (negative gravitaxis) after getting shaken in a test tube. When the antennal aristae were ablated this negative gravitaxis behavior vanished, but not the phototaxis behavior (flies fly towards light source). Removing also the second segment, i.e. where the JO is located, the negative gravitaxis behavior came present again. This shows that when JO is lost, Drosophila can still perceive gravitational force through other organs, for example mechanoreceptors on neck or legs. These receptors were shown to be responsible for gravity sensing in other insect species .

Silencing specific neurons

It is possible to silence selectively subgroups of JO neurons using tetanus toxin combined with subgroup-specific GAL4 drivers and tubulin-GAL80. The latter is a temperature-sensitive GAL4 blocker. With this it could be confirmed that neurons of subgroup CE are responsible for gravitaxis behavior. Elimination of neurons of subgroups CE did not impair the ability of hearing . Silencing subgroup B impaired the male’s response to courtship songs, whereas silencing groups CE or ACE did not . Since subgroup A was found to be involved in hearing (see above) this result was unexpected. From different experiment, in which the sound-evoked compound action potential (sum of action potentials) were investigated the conclusion was drawn that subgroup A is required for nanometer-range receiver vibrations as imposed by faint songs of courting males.

Figure 2: A) left: Neurons of different subgroups A-E are illustrated in the JO. right: The corresponding target zones of the subroups are shown in the AMMC. B) Simplified circuitry of the auditory (zone AB) and deflection sensitive (zone CE) system. These two systems separated similar as in vertebrates. Neurons of zone A have target zones in the AMMC, vlprs and SOG. Vlpr stands for ventrolateral protocerebrum, SOG for suboesophageal ganglion, A for anterior, D for dorsal, M for medial. (adapted from Figure 2.10 of ).

Origin of difference of the subgroups

As mentioned above the anatomically different subgroups of JO neurons have different functions . The neurons do attach to the same antennal receiver, but they differ in opposing connection sites on the receiver. Thus for e.g. forward deflection some neurons get stretched whereas others get compressed, which yields different response characteristics (opposing calcium signals). The difference for vibration- and deflection-sensitive neurons may come from distinct molecular machineries for transduction (i.e. adapting or non-adapting channels and NompC-dependent or not). Sound-sensitive neurons express the mechanotransducer channel NompC (no mechanoreceptor potential C, also known as TRPN1) channel whereas subgroups CE are independent of NompC . In addition JO neurons of subgroup AB transduce dynamic receiver vibrations, but adapt fast for static receiver deflection (i.e. they respond phasically) . Neurons of subgroups CE showed a sustained calcium signal response during the static deflection (i.e. they respond tonically). The two distinct behaviors show that there are transduction channels with distinct adaption characteristics, which is also known for the mammalian cochlea or mammalian skin (i.e. tonically activated Merkel calls and rapidly adapting Meissner’s corpuscles) .

Differences in gravitation and sound perception in the brain

Neurons of subgroups A and B target on one side zones of the primary auditory centre in the AMMC and on the other side the inferior part of ventrolateral protocerebrum (VLP) (see Figure 2B)). These zones show many commissural connections between themselves and with the VLP. For neurons of subgroups CE almost no commissural connection between the target zones were found, nor connections to the VLP. Neurons associated with the zones of subgroup CE descended or ascended from the thoracic ganglia. This difference in the AB and CE neurons projection reminds strongly on the separate vertebrate projection of the auditory and vestibular pathways in mammals .

Johnston’s Organ in honeybees

Solitary bee (Anthidium florentinum): the Johnston's organs on the head are head are clearly visible.

The JO in bees is also located in the pedicel of the antenna and used to detect near field sounds . In a hive some bees perform a waggle dance, which is believed to inform conspecifics about the distance, direction and profitability of a food source. Followers have to decode the message of the dance in the darkness of the hive, i.e. visual perception is not involved in this process. Perception of sound is a possible way to get the information of the dance. The sound of a dancing bee has a carrier frequency of about 260 Hz and is produced by wing vibrations. Bees have various mechanosensors, such as hairs on the cuticle or bristles on the eyes. Dreller et al. found that the mechanosensors in JO are responsible for sound perception in bees . Nevertheless hair sensors could still be involved in detection of further sound-sources, when the amplitude is too low to vibrate the flagellum. Dreller et al. trained bees to associate sound signals with a sucrose reward. After the bees were trained some of the mechanosensors were abolished on different bees. Then the bee’s ability to associate the sound with the reward was tested again. Manipulating the JO yielded loss of the learnt skill. Training could be done with a frequency of 265 Hz, but also of 10 Hz, which shows that JO is also involved in low-frequency hearing. Bees with only one antenna made more mistakes, but were still better than bees that had ablated both antennas. Two JO in each antenna could help followers to calculate the direction of the dancing bee. Hearing could also be used by bees in other contexts, e.g. to keep a swarming colony together. The decoding of the waggle dance is not only done by auditory perception, but also or even more by electric field perception. JO in bees allows detection of electric fields . If body parts are moved together, bees accumulate electric charge in their cuticle. Insects respond to electric fields, e.g. by a modified locomotion (Jackson, 2011). Surface charge is thought to play a role in pollination, because flowers are usually negatively charged and arriving insects have a positive surface charge . This could help bees to take up pollen. By training bees to static and modulated electric fields, Greggers et al. showed that bees can perceive electric fields . Dancing bees produce electric fields, which induce movements of the flagellum 10 times more strongly than the mechanical stimulus of wing vibrations alone. The vibrations of the flagellum in bees are monitored with JO, which responds to displacement amplitudes induced by oscillation of a charged wing. This was proven by recording compound action potential responses from JO axons during electric field stimulation. Electric field reception with JO does not work without antenna. Whether also other non-antennal mechanoreceptors are involved in electric field reception has not been excluded. The results of Greggers et al. suggest that electric fields (and with it JO) are relevant for social communication in bees.

Importance of JO (and chordotonal organs in general) for research

Chordotonal organs, like JO, are only found in Insecta and Crustacea . Chordotonal neurons are ciliated cells . Genes that encode proteins needed for functional cilia are expressed in chordotonal neurons. Mutations in the human homologues result in genetic diseases. Knowledge of the mechanisms of ciliogenesis can help to understand and treat human diseases which are caused by defects in the formation or function of human cilia. This is because the process of controlling neuronal specification in insects and in vertebrates is based on highly conserved transcription factors, which is shown by the following example: Atonal (Ato), a proneural transcription factor, specifies chordotonal organ formation. The mouse orthologue Atoh1 is necessary for hair cell development in the cochlea. Mice which expressed a mutant Atoh1 phenotype, which are deaf, can be cured by the atonal gene of Drosophila. Studying chordotonal organs in insects can lead to more insights of mechanosensation and cilia construction. Drosophila is a versatile model to study the chordotonal organs . The fruit fly is easy and inexpensive to culture, produces large numbers of embryos, can be genetically modified in numerous ways and has a short life cycle, which allows investigating several generations within a relative short time. In addition comes that most of the fundamental biological mechanisms and pathways that control development and survival are conserved across Drosophila and other species, such as humans.