Sensory Systems/Ants/Olfactory System

Olfactory System of Ants

Introduction

Ants are a very successful species, owing in large part to their intricate social organization and parsimonious array of sensory processing capabilities. As ants live in colonies of millions of members, solid communication abilities, such as signaling to other individuals the whereabouts and plentifulness of food sources or foreign colonies, are crucial. Keeping track of their environment allows ants to regulate their foraging activities. Ants also use their olfactory sensation to find back to their nest and use pheromone deposition to regulate colony-scale emergent behavior to find the shortest paths to food sources.

Olfaction

Olfaction in Ants is carried out by pheromones, small organic molecules that are produced by different glands, such as Dufour’s gland, the poison gland, the anal gland, and glands on feet, abdomen and thorax. These pheromones are used to exchange information about mating, predators, trail marking or food sources. Some ant species, such as the Pharaoh’s ant, have distinct pheromones of different valence and volatility. Pheromones can be either high or low volatility and attractant or repellant. This allows foraging ants to reinforce tracks to plentiful food sources in order to amplify rewarding paths and cut down attractance to scarce food sources. This way a pheromone network is created that is easily adaptable to changing environmental conditions. In a way ant colonies work like a single super-organism that can retain memory chemically with short and long term memory.

Olfaction in pathfinding

Figure 1. Schematic representation of maze linking food source to the nest via different path length.

An interesting perspective linking olfaction to pathfinding was elucidated by the following experiment on desert ants: Researchers marked the visually inconspicuous entrance to their nest (a small hole in the ground) by four odorants which were previously shown to bear no valence, neither attractive nor repellent. The four organic molecules were deposited at the corners of an imaginary square surrounding the nest entrance. Ants subsequently learned to associate the smells with their nest entrance, such that under test conditions (without any nest present) they would actively seek out the center of the odor square. However, when the spatial arrangement of the odors was modified, the ants no longer recognized it, suggesting that the olfactory memory they resort to for navigation has an essential spatial dimension. Finally, when one antenna was severed (ants possess two antennae equipped with olfactory receptors), ants also failed to recognize the odor square. This led researchers to conclude that ants “smell their scenery in stereo”

The ability to quickly find the shortest path to ephemeral food sources is a key survival factor for ants. When placed in an environment where the nest and a food source are connected by two paths of different length ants become increasingly biased towards the shorter path. In the beginning ants choose randomly between the two available paths, but ants that chose the shorter one are able to make the trip several times, thus leaving more chemoattractants on their path. The higher pheromone concentration will induce more ants to select the short path, thereby reinforcing the bias with every journey. This shows that a complex navigation problem can be solved by the colony-scale behavior emerging in a system with a single, positively valenced pheromone.  

Role and morphology of the antenna

Figure 2. Scanning electron microscopy image of ant head morphology

The major channel of sensory input for ants is the antenna. By moving it around, they can touch, taste, and smell everything within their reach. The function of these antennae goes well beyond taste and olfaction, though: the chemical cues picked up through them are key to several behavioral functions necessary for their survival. They include mate recognition and selection, discrimination between nestmates and foreigners, locomotion and exploring new grounds for food, communication, and defensive behaviors. There are also examples of subfamilies of ants (Dorylinae, Leptanilinae, Cerapachyinae) that are all blind and rely exclusively on chemosensory signals. It is therefore mainly through chemosensing that ants are able to form the highly efficient organized colonies they are known for. The ant antenna is unique in that it is bent, or “elbowed”. It consists of a long elongated basal segment called the scape, and three to eleven distal segments collectively called the funiculus, or agellum. The scape and agellum meet at an angle, therefore giving the ant antenna its characteristic bent shape. Each segment is called an antennomer, so the total antennomer count for ants is between four and twelve.

The antenna contains three types of sensory receptors: odorant receptors (ORs), gustatory receptors (GRs), and ionotropic glutamate receptors (IRs). GRs contribute to the ant's sense of taste and pick up a limited amount of pheromones, while IRs are narrowly tuned receptors whose main use is to detect poisonous and toxic compounds. Ant IR and GR levels are comparable to IR and GR levels in other insects; however OR levels, responsible for responding to odorants and a wider array of pheromones, are disproportionately high in ants and allow them to detect chemical substances at lower concentrations than other insects.

Neuronal morphology of the antenna

The sensory neurons are located on silia, hair-like structures on the antennae. Within those structures are the sensilium filters (also referred to as sensilia), a cuticular apparatus that contains a group of sensory neurons and their auxiliary cells. Each antennomer contains a few different types of sensilia and secretory gland cells and can be observed with scanning electron microscopy. The types of sensilia differ not only between ant species but are also specific to the gender and caste of the ant in question. Different types and functions of sensilia Based on the morphological features of the sensilia (for instance, the presence or absence of pores), one can speculate on their functions and how they serve a specific sensory modality. There are seven different types of sensilia each with their own types of inputs:

• 1. Sensilla chaetica: mainly related to mechanosensory inputs
• 2. S. trichodea: olfactory
• 3. S. trichodea curvata: olfactory, including pheromones
• 4. S. basiconica: olfactory
• 5. S. coeloconica: olfactory
• 6. S. ampullacea: has no pores and contains hygroreceptors that respond to humidity, thermoreceptors that respond to temperature, and specialized receptors that respond to CO₂ cues
• 7. S. campaniformia: olfactory

The physiological functions of these sensory neurons in ants have mostly been studied using bioassay techniques and electrophysiological recordings using a variety of different ant species. Again, not all sensilia are always present on an ant, but five dominant types are usually present for all: S. basiconica, S. chaetica, S. trichodea curvata, S. coeloconica, and S. ampullacea. They also all have a characteristic protruding pore plate (S. trichodea curvata) structure, which provides a higher absorption probability for odorous molecules, as opposed to a at plate. The large surface area found in S. basiconica (long pegs), S. trichodea curvata (long hairs), and S. coeloconica (large openings) could also favor the collection of more molecules . The major type of sensilia for both males and females are S. trichodea curvata, which serve an olfactory purpose.

Odorant Receptor Neurons

OR neurons are located on specialized olfactory sensilia. Each OR neuron usually expresses a single receptor, along with what is called an Orco (odorant receptor co-receptor), together forming a functional receptor unit. OR neurons convert chemical activity into neural signals sent to glomeruli in the antennal lobe. The sensilia present in a specific ant depends on the ant's species, caste and gender. Some OR gene family members are also enriched specifically in worker ant antennae. OR neurons have also been proposed to be the principle mechanism by which ants detect queen cuticular hydrocarbons, which in turn regulates worker-specific behaviors.

Sexual and caste dimorphism in sensilia relates to function

Colonies of ants consist of non-reproductive females and reproductive males and females. All worker, soldier, and queen ants are female. The few fertile male ants don't work in the colony and are only alive for a few months to fertilize the queen ant during nuptial flights. Sensilia are sexually and caste dimorphic: they differ depending on the sex and caste of the ant. Overall, males have about a third of the number of ORs found in females. Recent research has shown that males also have ORs specifically tuned to pheromones produced by the queen. Scanning electron microscopy studies on the red fire ant have given us some other interesting examples, for example male antenna have porous sensilia on all segments. Morphological differences in sensilia have been proposed to not only relate to, but determine ant functional needs. In other words, the absence or presence of sensilia sensitive to task-related odors and cues could constitute a possible mechanism by which task assignments are determined in a colony. For example, in S. invicta worker ants are polymorphic, with the total number of sensilia depending on club length, which constitutes a difference in sensitivity to olfactory signals.