This post is the fifth in a short blog series called “Know your Insect”. The images and descriptions are written by Entomology graduate students enrolled in a seminar of the same name.

By: Maridel Fredericksen

Like most other animals, insects have a central nervous system that helps them sense and respond to their environment. The central nervous system of insects (along with the rest of the arthropods and many other invertebrates) is called the ventral nerve cord, and it consists of a brain, a part underneath the brain called the sub-esophageal ganglion, and a cord that runs along the length of their body. This cord is similar to our spinal cord except that it runs down the front of their body instead of the back.  So the quickest way to get to the nerve cord of a cockroach (or any insect) is to cut open its belly!

Structure of Ventral Nerve Cord

If you actually did cut open a cockroach and take out its nerve cord, it would look a little like a string of beads. The “beads” are clusters of nerve cells called ganglia, and the string-like connections between them are called connectives. The ganglia and connectives can be arranged different ways in different insects. The very first insects had their ganglia roughly evenly spaced along the nerve cord, with one ganglion innervating (sending signals to) each body segment, including 3 thoracic segments and 8 abdominal segments (Niven et al., 2008). Some insects like bristletails still have nerve cords like this, but in many other insects some of the ganglia are closer together than others, and they may even fuse to form larger ganglia.

And you don’t even have to look at distantly related insects to see differences in their nerve cord structure; some closely related species of stingless bees have very different arrangements of their ganglia (Wille 1961), and there can even be differences between males and females of the same species (d’Herculais 1881) or between workers and queens (which are both female but develop differently) (Niven et al., 2008).

Function and Evolution
It’s not clear what all this diversity is for, but a general theory is that the insect nerve cord is arranged to minimize total wiring cost, balancing speed, space, and energy (Niven et al., 2008). Also, ganglia that are closer together can send signals to each other more quickly, and scientists think that some insects may benefit more from signaling quickly to adjacent neurons along the nerve cord, whereas others might prioritize signals sent to peripheral neurons (those that directly control motor movement) (Niven et al., 2008). Variation in life history traits could tip the balance one way or the other—for example, female conopid flies have an abdominal ganglion very far back in their bodies, whereas in males it is much farther forward. It is thought that this different arrangement could have evolved because females need fine control of complex muscles for egg-laying, whereas males do not (Streiff 1906).

 

The insect brain

While all ganglia on the ventral nerve cord contribute to organizing, managing, and synchronizing information that ultimately directs the insect’s behavior, the supraesophaegal ganglion (or brain) is the one that has been studied the most. And actually, the brain is made up of three fused ganglia that have different functions. The protocerebrum is the largest and most complex ganglion and includes the optic lobes (ol: Figs 2, 3) for vision and the mushroom bodies (cal: Figs 2, 3) which are involved with learning and memory and also processes olfactory (smell) information. The deutocerebrum includes the antennal lobes (me: Figs 2, 3), which receive information from the insect’s antennae. The tritocerebrum connects the brain to the rest of the nerve cord and also innervates some of the insect’s mouthparts.

Studying the insect brain

Early research on the insect nervous system included ablation experiments, where researchers would take out part of the brain (or cut off the whole head!) to see how this would affect the insect’s behavior. These experiments taught us a lot about how the insect nervous system works. For example, Donald Wilson found out that central pattern generators for locust flight originate in the thoracic ganglia, meaning that headless locusts can still fly (Wilson 1961a), and Kenneth Roeder found out that headless cockroaches can learn to avoid an electric shock (Roeder et al., 1960).

Nowadays, much of the research on insect brains is done with the help of advanced microscopy techniques such as CLSM (confocal laser scanning microscopy). For example, researchers have used these techniques to show that ants’ brain morphology can change over their lifetime as they age and switch tasks in the nest (Gronenberg et al 1996), and that whether ants are large or tiny, the density of their synapses (so how tightly packed their neurons are) in certain brain regions is similar (Groh et al 2014).

My research: zombie ants

Part of my research involves using CLSM to study brain morphology of ants. Specifically, I am trying to find out if a behavior-controlling parasite changes the way an ant’s brain looks under the microscope. I will take images of healthy ant brains and compare these to brains of ants infected with the fungal parasite, Ophiocordyceps unilateralis. This fungus causes infected ants to leave their colony and bite onto the underside of a leaf or twig before they die. This behavior is adaptive for the fungus because being above the ground and close to ant foraging trails allows it to grow out of the ant’s head and shoot spores to the ground below where it will infect other ants (Andersen et al. 2009; Hughes et al. 2011). I will also use a fungus that kills the ants but does not manipulate them, to find out what is special about the brain of a manipulated ant compared to one that is just sick. From this research, I hope to eventually understand how the fungus controls the ant’s behavior. And if we understand how a brain can be manipulated, then we can better understand how normal brains work.

This post is the sixth in a short blog series called “Know your Insect”. The images and descriptions are written by Entomology graduate students enrolled in a seminar of the same name.

By: Luca Franzini

Insects live in a world dominated by scent. Chemicals play a fundamental role in most aspects of their complex, fascinating lives and are frequently actively produced by the insects themselves. For example, a queen honey bee rules over tens of thousands workers just by releasing a single, specific compound, the queen mandibular pheromone 9-oxo-(2E)-decenoic acid, which suppresses the ovarian development of her subjects (Hoover et al., 2003). Furthermore, females of several species of moth will attract males over long distances by emitting species-specific sex pheromones. Only males from their species will come to court them. However, what do all these chemicals, eliciting such complex behaviours, have in common? Well, they are all produced by glands.

Glands in insects are found spread across their whole body, in their mandibles, in their brain, in their gut. Their role is as varied as insects themselves, and range from mate attraction to defense to complex social communication. While most glands are found across all insect orders, several taxa will often possess their own unique glands, which will also vary in their uses and contents across the various species.

The Dufour’s gland is found only in Hymenoptera, in the suborder Apocrita. It was first described by Léon Jean Marie Dufour in 1841 and for a long time it was known as the alkaline gland. This name derived from the misconception that, together with the acid gland (now known as the venom gland), it was associated with stinging in bees and wasps. It was thought that its secretions activated the venom, by mixing with those from the acid gland, or that they would neutralize the acidic ones left inside the sting after a bee or wasp had stung an enemy.

While it is true that the Dufour’s gland is associated with the sting apparatus in Aculeata, it has no role in stinging or venom production and its duct opens into the dorsal vaginal wall (Billen, 1987). The only exception to this is found in the family Formicidae, where the gland opens into the sting bulb, but still with no involvement with the main function of the sting itself (Billen, 1987). It develops as an invagination of the dorsal vaginal wall (membrane connecting the ventral margins of the dorsal valves) and like the other accessory glands of the sting apparatus, the venom gland and the spermatheca, it is of ectodermal origin. Shaped like a long, slender tube, it displays conspicuous folding of its epithelium, which gives the gland an accordion-like appearance (Billen, 2006). The gland cells are lined with cuticle, as in other class I epidermal glands, through which their secretions must go through to reach the lumen.

 

The gland content varies across species, but in most taxa it is mainly composed by long-chain saturated and unsaturated hydrocarbons, a condition that appears to be the ancestral one for the Dufour’s gland (Mitra, 2013). These compounds are not synthesised by the gland itself, but are produced by specialised secretory cells called oenocytes (Makki et al., 2014).

These cells are found in clusters in the abdomen and sometimes in the thorax of most Pterygota and are often in association with fat bodies. They possess a well-developed smooth endoplasmic reticulum and play a key role in lipid metabolism, detoxification, and synthesis of hydrocarbons and other lipid-derived substances (Makki et al., 2014). From the oenocytes, these compounds are released into the haemolymph from which they reach the Dufour’s gland and the cuticle where they create a waxy layer. This coating on the cuticle is usually a mixture of hydrocarbons, sterols, fatty acids, fatty alcohols, and wax esters. The function of this coating is primarily as a barrier against desiccation. Moreover, the profile created by these chemicals is usually species-specific and it is involved in chemical communication (Mitra, 2013). Interestingly, the content of the Dufour’s gland has been found to match closely the composition of this cuticular waxy layer in a variety of species, including bumble bees and the social wasp Ropalidia marginata (Oldham et al., 1994).

The cuticular hydrocarbon profile of social Hymenoptera is of fundamental importance for in-nest dynamics, as it represents one of the main components of their complex chemical communication system. Not only does the profile tend to be species-specific, but it is often also colony specific. This provides a barrier against potential intruders. Nonetheless, communication is just one of the many uses for the Dufour’s gland, some of which can be quite peculiar. For example, in parasitic wasps of the superfamily Ichneumonoidea, female wasps will use secretions from this gland to mark their hosts after laying their eggs in it to avoid competition from related individuals (Mitra, 2013).

In social wasp from the subfamily Stenogastrinae, the gland secretes a white, gooey mass, called abdominal substance, which serves a variety of functions (Turillazzi, 1991).
It is placed around the pedicel of their nests where it acts as an ant repellent, but it is also used for liquid food storage and, most importantly, as the base onto which the eggs are laid.
In solitary bees, in particular in ground-nesting species (e.g., Colletidae, Andrenidae, Halictidae), the gland became enlarged during the construction of the brood cells, occupying large part of the metasoma (Lello, 1976). This is due to the fact that the Dufour’s gland in these species secretes macrolytic lactones, a form of natural polyesters, which are used in brood cells lining (Hefetz, 1987). This cellophane-like coating prevents desiccation, flooding of the cells, and it has antimicrobial properties.

Hypertrophism of the Dufour’s gland is also found in dulotic ants (D’Ettorre et al., 2000). Also known as slave-making ants, they raid other species’ nests, capture their brood and after bringing it back to their nest they enslave the emerging workers. To help in their raids, they rely on the propaganda substances produced by the Dufour’s gland, which mimic the alarm pheromone of the host and will cause alarm among the defending workers, generating panic and allowing the raiders to pillage the nest (Buschinger, 2009).
Hacking into another species’ communication system is more common than one would think, and it is rarely done without ill-intent. For instance, males of several species of the kleptoparasitic genus Nomada will, while mating, coat the female with secretions from their mandibular glands (Tengö and Bergström, 1977). These secretions mimic those of the Dufour’s gland of the host species and help the female avoid confrontation with the host while entering its nest (Mitra, 2013).

Another well-known example of this kind of hacking is social parasitism, a fascinating lifestyle where a species will exploit the social structure of another for its own benefit, typically to rear its brood.

Social parasites are common within the various groups of social Hymenoptera, and bumble bees are no exception. There are about 250 species of bumble bees (Apidae,Bombus), about 30 of which are socially parasitic. With the exception of two species, all the remaining parasitic ones can be found grouped in the subgenus Psithyrus. Members of this subgenus, commonly referred to as cuckoo bumblebees, are morphologically and physiologically adapted to a parasitic lifestyle and rely on various chemical strategies to infiltrate their host colonies and usurp them (Fisher and Sampson, 1992).

Chemical mimicry is perhaps the most fascinating among the strategies used by these parasites. It has been demonstrated that some species of cuckoo bumblebees actively mimic the cuticular hydrocarbon profile and Dufour’s gland content of their host species (Martin et al., 2010). This allows the usurper to take over the role of dominant reproductive female, the queen, with relative ease and maintain it long enough for its offspring to fully develop inside the host colony. When this mimicry is not possible or achieved, as in the case of generalists parasitising more than one host species, some species will sneakily rely on worker repellents, like dodecyl acetate, secreted from their Dufour’s gland (Zimma et al., 2003; Martin et al., 2010).

My research aims at further investigating the chemical strategies employed by cuckoo bumblebees during their rule of a host colony. I will look into various aspects of the usurpation process, from chemical mimicry to the use of worker repellents, in a wider range of species and I will investigate how the different developmental stages and sexes of the social parasite cope with life inside another species’ nest. Even though these are just few of the questions left unanswered about the cuckoo bumblebees’ life history, and only a small portion of those I would like to answer with my future research, I hope my work will shed some light on the still poorly-understood life of these amazing insects.

This post is the final in a short blog series called “Know your Insect”. The images and descriptions are written by Entomology graduate students enrolled in a seminar of the same name.

By: Yuhao Huang

What are insect salivary glands?

Insect salivary glands are several exocrine glands that are associated with insect oral cavity and produce secretions (typically saliva) mixed with food during ingestion. There are basically four types of glands— hypopharyngeal glands, maxillary glands, mandibular glands, labial glands—which can be called salivary glands in insects. But generally these glands do not all occur at the same time in an insect.

Hypopharyngeal glands

For hypopharyngeal glands, they normally occur in Hymenoptera, like honey bees. In addition to the salivary enzymes they produce, these glands also yield secretions to feed the larvae.

Maxillary glands

For maxillary glands, they are often found in some non-insect hexapods, like Protura and Collembola, and in some larval Neuroptera and Hymenoptera. These glands are involved in the lubrication of mouthparts (Francois et al. 1986).

Mandibular glands

For the mandibular glands, they can be found in many Hymenoptera and Lepidoptera larvae. In some hymenopterans, mandibular glands excrete alarm pheromones while in some Lepidoptera larvae they function as part of predigestion system and chemical recognition of plants and microorganisms (Maria et al. 2012).

Labial glands

For the last one—labial glands, they are almost present in all insects, except in Coleoptera. Their structure differs among diverse orders and in some Lepidoptera larvae, they are called silk glands. Generally, they are the main functional salivary glands because they produce most of the saliva for insects.

Saliva

Normally, the main products of salivary glands are saliva which have been demonstrated having diverse roles. For example, it has been found that larval Helicoverpa zea saliva’s protein—glucose oxidase—can suppress tobacco plant Nicotiana tabacum’s induced defenses—the production of nicotine (Musser et al. 2012).  In blood-feeding insects like mosquitoes, one of the critical functions of the saliva they produced is to prevent blood clotting (Ribeiro et al. 2003).

My research

One of my research projects is to probe the effect of secretions of insect herbivores’ labial glands on plant’s induced defenses. Most of the study about how herbivore’s saliva affect plant-induced defenses concentrate on the labial glands.

However, the roles of mandibular glands on plant defenses has been largely neglected,  including their accessory glands, like Lyonet’s gland. So the exploration their functions will facilitate our understanding of how insect herbivore digesting food and countering host plant defenses.

This post is the fourth in a short blog series called “Know your Insect”. The images and descriptions are written by Entomology graduate students enrolled in a seminar of the same name.

By: Kyle Burks

The insect circulatory system is very different from that of mammals. Insects do not have lungs and they do not circulate oxygen through blood. Instead, they have a series of openings along theirs sides called spiracles. The spiracles open up into a system of air-filled tubes, the tracheae. Tracheae terminate in very fine branches called tracheoles which supply oxygen directly to every cell.

Insects can have up to two spiracles on the thorax and eight on their abdomen. Thoracic spiracles have a different morphology from abdominal spiracles. Abdominal spiracles can have up to two muscles that open and close them (dilator and occlusor; Fig. 1), though thoracic spiracles usually have a single closer (occlusor) muscle (Figs 2-5). Muscles are not necessary for opening spiracles, though. When relaxed, the spiracles are at least partially open (Chapman 2012). This prevents suffocation if consciousness is lost.

Thoracic spiracles have diverse morphology (Hassan 1944). One of the more common systems, at least among Hymenoptera, is a single muscle which tugs on a flexible ring at the beginning of the trachea. We took images of the first thoracic spiracle of several undescribed species of Nearctic Dendrocerus (Hymenoptera: Megaspilidae) using Confocal Laser Scanning Microscopy (CLSM). CLSM excites the specimen with a laser and captures the fluorescence that results.

 

CLSM splits the fluorescence into multiple channels based on their wavelength. Since different kinds of structures auto-fluoresce in different wavelengths, the color in the CLSM image encodes useful information. Soft structures like muscles, fat bodies, glands, and conjunctiva appear green, while harder structures such as the cuticle or chitinous structures appear red with 488 nm excitation laser. Areas with an abundance of the elastic protein resilin fluoresce the most strongly with the 405 nm laser in the blue range.

Between the opening of the spiracle and the start of the trachea, there can be up to two chambers with a hardened, thickened wall called atria (singular: atrium; Tonapi 1957). Atria are common among insects. In the CLSM images of Dendrocerus, we can see that a muscle inserts between the atrium (atr: Figs 2, 4) and the trachea (tr: Figs 2, 4). The bright blue in this region is the flexible tendon of the occlusor muscle and the flexible ring that closes the trachea.

The group of wasps that I study, Dendrocerus, sometimes have enormous atria. Why might they have such large atria though? What functions might a large atria such as this be capable of? Dendrocerus are extremely small wasps, and as such, are capable of being trapped in water droplets. This is also a danger experienced by some thrips, which have a modification to the outside of their spiracles which trap air bubbles around the the spiracle to prevent accidental drowning (Wiesenborn 2014). Could this explain the large atria of Dendrocerus? Might it act as an emergency air reservoir? Perhaps it may help maintain humidity instead. By having a large, round forechamber the tracheal system does not open up directly to the outside, which could help prevent water loss.

Do you have any ideas on what the function of these large atria could be? Leave a comment!