Insects feel through their antennae

Insects perceive their surroundings through different organs, among which antennae are some of the most important. Antennae appear in a lot of incredibly diverse shapes and sizes, and every group of insects develops one or more models. We encourage you to know more about their origin, functions and diversity through this post.

The origin of antennae

Antennae are paired sensorial appendages located in the anterior parts of insects’ body. Except for chelicerates (spiders, scorpions…) and proturans (non-insect hexapods), all arthropods, either crustaceans, hexapods (diplurans, springtails -Collembola- and insects), myriapods (centipedes and millipedes) and the extinct trilobites, have antennae when being adults.

In crustaceans, antennae appear in the two first head segments: a first pair known as primary antennae or antennules, and a longer second pair known as secondary antennae or just antennae. Usually, secondary antennae are biramous (that is, they have two main branches), even though some crustaceans have undergone ulterior modifications so antennae appear as uniramous appendages (with a single branch) or even get reduced.

Types of antennae in crustaceans. Picture obtained from Wikipedia (link).

However, the rest of arthropods only have a single pair of uniramous antennae. Hexapods (like insects), which seem to be closely related to crustaceans according to the pancrustacean model, seem to have just preserved the secondary pair of antennae typical of crustaceans.

According to some authors, antennae appear to be true appendages; that is, they would start to develop during the embryological development from a head segment the same way legs do. However, this segment would have evolved into a reduced and inconspicuous piece, now being unappreciable. Moreover, antennae can also regenerate like legs.

How do insects feel through antennae?

So, what does this title exactly mean?

Antennae are microscopically covered with tiny hairs known as sensilla, which are not related with hairs found in vertebrates since they are made of chitin (as the rest of insect’s cuticle) instead of keratin.

Picture above: antennae under electronic microscope. Picture below: detail of the sensilla. Both images taken from cronodon.com.

Despite being almost identical at the first sight, there are different types of sensilla: chemoreceptorial sensilla have an inner channel through which suspended molecules enter (e.g. pheromones), while mechanoreceptorial sensilla are retractable and move at the slightest pressure or when the insect changes its position with respect to the ground (in this case, these are called proprioceptor sensilla).

So, insects taste, smell, touch and communicate in part through antennae, thus allowing them to gather information about food sources, potential mates (pheromones), enemies, dangerous substances (e. g. a poisonous plant), nesting places and migratory routes (as in the case of the monarch butterfly). Other organs, such as legs, palpi and even the ovipositor (organ for laying eggs) sometimes have sensorial cells.

Inside and in the base of sensilla there are sensorial neurons connected to the insect’s brain; specifically, a brain region known as deutocerebrum. In chemoreceptorial sensilla, molecules bind with specific receptors that send nervous signals to the antennal lobe through the sensorial neurons. This lobe is somewhat like the olfactory bulb found in vertebrates.

Types of antennae in hexapods

Except for the proturans, which are wingless hexapods, diplurans, springtails (collembola) and insects develop different types of antennae. These are divided in two main groups:

• Segmented antennae: springtails and diplurans. Each segment has an own set of muscles that moves it independently from the rest of the antenna.
• Flagellate antennae: insects. Just the first segment located at the base of antennae in contact with the insect’s head (the scapus) has an own set of muscles, so the antennal movement depends entirely on this segment.

Parts of insects’ antennae

The three basic segments of insects’ antennae are the following:

Antenna of an inquiline wasp belonging to the genus Synergus (Hymenoptera). Picture by Irene Lobato.

1) Scape: basal segment that articulates with the insect’s head and the only one that has an own set of muscles. The scape is mounted in a socket called torulus.

2) Pedicel: the second antennal segment or the one that comes just after the scape. This segment has a relevant role since it contains the Johnston’s organ, which is a collection of sensory cells. This organ is absent in non-insect hexapods (springtails, diplurans).

3) Flagellum: the rest of antennal segments that form the antennae, which are individually known as flagellomeres. These flagellomeres are connected by thin membranes that allow them to move as a whole despite not having muscles.

Thousands of antennae!

From this basic pattern (scape + pedicel + flagellum), each group has developed numerous antennal models based on their lifestyle:

• Aristate

These are very reduced antennae with a pouch-like shape and a small bristle that emerges from its third modified segment.

Example: a very extended model among flies (Diptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a fly of the family Sarcophagidae by JJ Harrison, CC 1.0.

• Capitate

Capitate antennae have a club or knob at their ends.

Example: usually found in butterflies (Lepidoptera) and in some beetles (Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; middle: picture of a beetle of the species Platysoma moluccanum by Udo Schmidt, CC 2.0; left: a butterfly, public domain.

• Clavate

Unlike the capitate ones, clavate antennae get progressively thicker in their ends.

Example: moths (Lepidoptera), carrion beetles (Silphidae, Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; right: beetle of the species Thanatophilus sinuatus (Silphidae) by Wim Rubers, CC 3.0.

• Filiform

This is the simplest model of antennae: long, thin and made of equally sized and shaped segments.

Example: cockroaches (Blattodea), crickets and grasshoppers (Orthoptera), longhorn beetles (Cerambycidae, Coleoptera), bugs (Heteroptera).

Left: picture by M. A. Broussard, CC 4.0; right: cockroach of the species Periplaneta americana by Gary Alpert, CC 3.0.

• Flabellate

These are quite similar to pectinate and lamellate antennae (see below), but with thinner and flattener segments that make them to look like a folding paper fan; also, these thin projections occupy all the antenna, and not only the terminal segments as in lamellate antennae. This model is found in males of some insects, thus having a large surface for detecting pheromones.

Example: beetles (Coleoptera), wasps (Hymenoptera) and moths (Lepidoptera).

Beetle male of the genus Rhipicera. Picture by Jean and Fred, CC 2.0.

• Geniculate

These are bent, almost like a knee joint. The first antennal segment (scape) is usually located before the joint. The rest of segments together are known as funicle.

Example: some bees and wasps, especially in chalcid wasps (Hymenoptera), weevils (Curculionidae, Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a parasitoid wasps of the species Trissolcus mitsukurii, public domain.

• Lamellate

The terminal segments enlarge to one side in form of flat and nested projections, thus looking like a folding fan.

Example: beetles of the family Scarabaeidae (Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a beetle of the family Scarabeidae, public domain.

• Moniliform

Unlike filiform antennae, the segments of moniliform antennae are more or less spherical and equally sized, thus giving these antennae a string of bead appearance.

Example: termites (Isoptera), some beetles (Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a termite by Sanjay Acharya, CC 4.0.

• Pectinate

Segments have a lateral projection, so they look like combs.

Example: sawflies (Symphyta, Hymenoptera), parasitoid wasps (Hymenoptera), some beetles (Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a beetle of the family Lycidae by John Flannery, CC 2.0.

• Plumose

Plumose antennae look like feathers as their segments have numerous thin branches. Having a bigger antennal surface allows them to detect more suspended molecules, like pheromones.

Example: mosquito (Diptera) and moth (Lepidoptera) males.

Left: picture by M. A. Broussard, CC 4.0; right: moth male of the genus Polyphemus by Megan McCarty, CC 3.0.

• Serrate

Each segment is angled or notched on one side, thus making these antennae to look like saws.

Example: some beetles (Coleoptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a beetle of the family Chrysomelidae by John Flannery, CC 2.0.

• Setaceous

These antennae are bristle-shaped, being thinner and longer in their ends. They are quite similar to filiform antennae, but thinner.

Example: mayflies (Ephemeroptera), dragonflies and damselflies (Odonata).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a dragonfly, public domain.

• Stylate

Similar to filiform antennae, but the terminal segments are pointed and slender, looking like a style. The style can either have bristles or not.

Example: brachycerous flies (Diptera).

Left: picture by M. A. Broussard, CC 4.0; right: picture of a brachycerous fly of the family Asilidae by Opoterser, CC 3.0.

You can read more about the different antennal models here and here, or take a look to the antennal gallery by John Flannery.

Main picture by Jean and Fred, CC 2.0.

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If you know more antennal models or curious facts about insects’ antennae, feel free to share it with us by leaving a comment below!

How do insects communicate?

How do ants know what path to follow? Which mechanisms do some male and female moths use to meet each other when located far away? As humans along history, insects have developed different ways to communicate with each other.

Do you want to know how and for what purpose do insects communicate by all its senses? Keep reading!

Insects language

Communication is defined as an exchange of information between two (or more) individuals: the one/s that transmits the message (emitter) and the one/s that receives and processes that message. While in humans communication passes through a long learning process, in insects the same process tends to be an inborn mechanism: each newborn individual has an specific vocabulary shared only with organisms of its own species.

On the other hand, we tend to see communication as an obvious process (if the emitter says “Thank you!” we expect the receptor to say “You’re welcome” in return). In insects, likewise in other animals, communication can take place in a way that information can’t be appreciated for us (humans).

Thus, it’s be better to say that communication is an act or condition of any part of an organism that alters the behavior of another organism. What does it means? That the emitter insect sends a missage to the rest of organisms by doing some action (e.g. an acoustic signal) or maybe by developing some physical trait which informs the rest of individual of some stuff (e.g. the color pattern of wings of some butterflies), in order to induce some answer or changes on the receptors that would benefit one or both of them.

Why do insects communicate?

Insects communicate both with organisms of the same species (intraspecific communication) and directly or indirectly with organisms of other species (interspecific communication) for many reasons:

• Reproduction: to look for a mate, courtship…
• To identify members of the same species or even to warn other organisms of its own presence.
• To localize sources of recourses : food, nidification places,…
• As an alert signal towards potential hazards.
• To defend territory.
• As a way to camouflage or to mimic other organisms (Do you want to learn more about animal mimicry? click here!).

Language through senses

Insects use almost all senses to communicate. Along this section, we’ll analyze one by one all communication systems that insects developed through the “five sense”, just like some of the flashiest examples.

Tactile communication: “The touch”

Tactile communication in insects would be equivalent to the sense of touch in vertebrates. Although nervous system in insects is underdeveloped compared to the one of vertebrates, tactile communication is based on the same principle: it must be some type of direct or indirect physical contact between the emitter of the message and the receptor.

• “Tandem running”: Follow the leader!

Since long ago, we know that ants walk in line one after another because some of them leave a chemical track that the rest of individuals follow to not get lost. But, aside of emitting these chemical signals, some ant species seem to establish an strategic physical contact system known as tandem running: the ant located behind touches the abdomen of the one that is immediately before it (the leader) with its antennae; moreover, if the leader stops feeling the antennae of the one behind, the leader will turn and wait for the one that follows it.

“Tandem running” steps observed in ants (it has also been studied in some termites species). Image source: link.

This video from Stephen Pratt Youtube channel shows two ants performing this kind of contact known as “tandem running”:

• Dancing bees

Honey bees (Apis mellifera) perform dances to show other members of their colonies where nectar is located (direction and distance) and also if it has a high quality. Bees dance inside their hives, so this performance takes place in deep darkness. So, you’ll ask yourself: why dance if no one sees you? Because the sense of sight is not necessary in this case to transmit the information: the rest of bees don’t perceive the movements in essence, but only the vibrations the dancing bee transmits trough all the hive will moving.

Look at these dancing bees! (video from Ilse Knatz Ortabasi Youtube channel):

Chemical communication: “smell and taste” 

Chemical communication is probably the most extended communication mechanism among insects. In this type of communication, the emitter scatters chemical substances at the environment which are detected by other organisms. There exists a lot of types of chemical substances: pheromones (for finding a mate), allelochemicals (as alarm signals, as a defensive system…), etc.

Even more important than how they scatter those substances, is the system they use to detect them: insects have more or less specialized receptors located on their antennae, their legs, etc. We can say they can savor and smell these substances with almost all parts of their body!

• Love gives you wings…and pheromones!

Females of some moth species emit pheromones that can be detected even by male moth located kilometers away. This is the case of Small Emperor Moth females (Saturnia pavonia), which attract males located almost 16km away.

Saturnia pavonia male (above) and female (below). Picture by Stephen Dalton ©.

• Your smell betrays you!

Communication can take place among insects of the same or different species. Euclytia flava is a bedbug parasitoid (learn more about parasitoids here) that detects its hosts by the way they smell: more accurately, by detecting the chemical substances that the hosts emit (these type of substances that benefit the receptor but not the emitter are known as kairomones).

Euclytia flava (Copyright © 2013 Christopher Adam).

Auditory communication: “the hearing” 

Insects emit a wide variety of sounds in different frequencies, amplitude and periodicity, and each species has a very well defined pattern. In fact, only by registering and analyzing insect’s sounds we can identify the species that has emitted them.

While humans can detect sounds in a range from 20 to 20.000Hz, insects can emit and detect sounds above this range (some crickets can produce ultrasounds above 80.000Hz).

• The summer sound

Cicades are amazing for many reasons: they remain more than 17 years in a nymph state underground until they reach adulthood and also emit a wide range of singings from sunrise to sunset during summer months. They emit these sound by stridulatory organs located in the abdomen, and are received by an auditory organs located on their legs or thorax.

Listen to this cicade singing! (Dangerous insects planet Youtube channel). Can see how its abdomen vibrates?

Some cicades are able to emit sounds that exceeds 120 decibels (they almost reach the human ear pain threshold!). However, some small cicade species emit sounds in a so elevated frequency that can’t be listened by humans, but that could be painful for other animals.

The sounds of cicades have many purposes, although they use it specially for finding a mate or to delimitate their territory.

• “I’m all antennae”

Some studies reinforce the idea that males of some mosquitoes species have a higher sensibility in their antennae to detect the vibrations emitted by the beating of female wings through the air.

Electron microscopy image of a male mosquito in which we can appreciate its feathery antennae; these pilosities heighten its sensibility (This image has been taken with EVO® MA10; picture by ZEISS Microscopy, CC).

Visual communication: “The sight”

Visual communication in insects takes place by two main systems: body color patterns and light signals (bioluminescence).

Each species has specific color patterns, which can be useful for identifying members of the same species, also to attract a mate o even to alert other organisms about its dangerousness (aposematic mimicry; learn more about it here), or to drive away predators. On the other hand, there are also species that emit light signals to attract other specimens (e.g. fireflies or beetle from Lampyridae family).

• Eyes…or only spots?

Caligo memnon, with its spots that resemble two big owl eyes and witch allow them to drive away predators (Picture by Edwin Dalorzo, CC).

• Lights in the dark
  

Fireflies are the most common example of communication mediated by bioluminescent signals, but there exist more insects which are able to emit light:

The click beetle (Pyrophorus Noctilucus) has two small bioluminescent organs located behind its head. The light of these organs get more intense when being menaced (Image source: firefly.org).

Larvae or larviform adult females from the beetle genus Phrixothrix emit two types of light: green and red. They emit red light by two organs located in their heads only when they feel menaced in order to alert other larvae about the presence of predators (image source: firefly.org).

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As you see, insects communicate in some different ways. Do you dare to discover how do insects that live near you communicate?

References

• Gopfert M.C; Briegel H; Robert D. (1999). Mosquito Hearing: Sound-Induced Antennal Vibrations in Male and Female Aedes Aegypti. The Journal of Experimental Biology. 202: 2727-2738.
• J.R. Aldrich, A. Zhang (2002). Kairomone strains of Euclytia flava (Townsend), a parasitoid of stink bugs. Journal of Chemical Ecology, Volume 28, Issue 8, pp 1565-1582.
• Nigel R. Franks, Tom Richardson (2006). Teaching in tandem-running ants. Nature 439, 153.
• Insectos: la mejor guía de bichos. Parragon Books Ltd.
• Hometrainingtools.com: insect communications
• www.cals.ncsu.edu/course/ent425/tutorial/Communication/
• Firefly.org

Main image by Radim Shreider © (National Geographic Photo Contest 2012).