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Evolution, Anatomy and Physiology, Mimicry
What makes a bee a bee? Learn how anatomy helps illuminate the intricacies of bee behavior and pollination and how to identify a bee.
Much of the introductory coursework medical students encounter centers around understanding human anatomy.
In fact, a robust understanding of anatomy is so fundamental for doctors that a year's worth of undergraduate anatomy and physiology coursework is required before even starting medical school. Knowing anatomy allows doctors to understand how different organ systems, behaviors, and diseases interact with your body and inform the best treatments.
Of course, entomologists may not be treating insects for illness as often as doctors treat humans — although there is a substantial history of disease treatment and prevention in managed pollinators. As with humans, insect anatomy and physiology determine how they interact and respond to their environment. In contrast to human anatomy, insect anatomy has another important purpose – to provide a basis for distinguishing among thousands of different species. While scientists often use modern molecular biology techniques to identify species, identification based on anatomy remains one of the most tried and true methods.
In this module, we will focus on the basic anatomy of adult mason bees. Many of the concepts and much of the vocabulary covered here are essential for understanding mason bee behavior and ecology and will be referenced throughout the other modules. After covering the topics in this module, students will be able to label the major anatomical structures of insects and describe the function of some of the more specialized features of bees.
Figure 1. All bees, and most insects with some exceptions, share a basic anatomy. Knowing these structures can help us understand a bee’s ecology and evolution
The three functional regions of an insect's body are the head, thorax, and abdomen. On the head, there are two large compound eyes, which are primarily responsible for sight, including for finding suitable nests and food. Three smaller simple eyes called ocelli are located in a triangle pattern on the top of the head. The visual information gleaned from ocelli is extremely low resolution compared to compound eyes, but they are exceptionally sensitive to light and can be used to help orient bees relative to the horizon during flight and monitor light intensity corresponding to the time of day. Between the compound eyes and below the ocelli are the antennae, which are highly sensitive chemosensory organs that play a vital role in detecting pheromones during mating or in some species to identify their nests, find food, detect pathogens, and numerous other tasks.
Figure 2. Bee mouthparts are complex, but all component works together to facilitate feeding and collection of pollen and nectar (Figure: Museum of the Earth, PRI).
Bee mouthparts are made up of many specialized structures that work in tandem to allow bees to feed on pollen and nectar and collect and shape mud, leaves, and other nesting materials. The two most notable structures are the mandibles (jaws), and the glossa (tongue) which bees insert into flowers when collecting nectar (Figure 2). A video showing how the individual mouthparts allow a bee to feed on floral resources can be found at youtu.be/xoCn9XDVI0A
One of the primary functions of the thorax is facilitating locomotion. All six of the bee's legs and four wings attach to the body in this region. From front to back, we call each pair of legs forelegs, midlegs, and hindlegs, and the two sets of wings are fore- and hindwings. You can distinguish between true bees and bee mimicking flies by counting the number of wings; flies only have one pair.
Figure 3. Scanning electron micrograph (SEM) of the branched setae
characteristic of bees (Image: Megan Asche, macronature.com).
The abdomen has fewer large and unique external features than the other two body regions; however, you may notice a stinger (usually rarely used by solitary bees) and genitalia. The anatomy of genitalia, much like that of mouthparts, is often used by entomologists to distinguish between species. The undeniable importance of eating and reproduction means that these structures face intense adaptive pressures leading to unique differences between closely related species.
Another unique characteristic of bees is that they are often, but not always, covered in a furry coat of small hairs called setae. In bees, these hairs are branched and aid in collecting pollen (Figure 3). Many setae serve an important role as sensory organs. These sensillae are highly specialized for various roles. There are mechanoreceptive sensillae for detecting mechanical stimuli, chemoreceptive sensillae for taste and smell, thermoreceptive sensillae for detecting temperature, and even hygroreceptive sensillae for detecting humidity.
Most importantly for bees, including mason bees, there are specialized setae for capturing and transporting pollen. These specialized setae form a structure called the scopa in female pollen-collecting (non-parasitic) bees (Figure 4). The scopa is usually located on the legs but in some bees, including mason bees, it is located on the underside of the abdomen. Bees use the scopa to collect pollen by packing it into the dense mat of hair so they can transport it back to their nests. Note that in some bees (such as Andrena) the scopa can consist of hairs on nearly all the leg segments, the posterior region of the thorax, and the underside of the abdomen, so it is very widespread.
Figure 4. Female Osmia conjuncta with its anatomical features labeled.
Note the scopa on the underside of the abdomen (Image: USGS BIML)
Not all bees carry pollen and nectar in large masses affixed to the side of their legs. In fact, only one group of bees (the corbiculates, which include honey bees and bumble bees) carry pollen this way. Corbiculate bees mix pollen and nectar into a sticky mass, which they hold in place on their legs in a specialized structure called a corbicula or pollen basket. Other bees use the scopa, as described above, to transport pollen. The brood parasites generally have no specialized anatomy for transporting pollen, so they lack a scopa entirely. Remember also that only female bees have a scopa — males never transport pollen.
Compound eye - A "high resolution" eye made up of many repeating units.
Corbicula - A specialized structure, sometimes referred to as a pollen basket, that is found on some bees' hind legs, including honey bees and bumble bees, and is used for transporting pollen.
Ocelli - Simple eyes that are highly sensitive to light but have low visual acuity.
Scopa - Specialized setae used for pollen collection.
Sensilla - Specialized sensory setae.
Setae - External hair-like structures on insects.
Is it a bee? — Identification and Anatomy
Using the provided images of bees, wasps, and flies, distinguish between these groups and label the major components of bee anatomy.
It can be challenging to tell the difference between bees and their close relatives, wasps and, complicating it further, many other insects (including flies and beetles) will mimic bees and wasps in order to trick humans and predators alike. Knowing basic bee anatomy will help you distinguish between these groups. However, beyond identifying bees, a bee's anatomy can tell us a lot about its ecology and evolutionary history. Throughout this program, we will return to the fundamental ideas conveyed in this module as we explore the nesting and foraging behaviors of mason bees and mason bees as pollinators.
Part 1: Bee Anatomy
Label the mason bee diagram with the following parts:
- Fore- and hindwings
- Compound eyes
- Scopa (if present)
- Head, thorax, and abdomen
Part 2: Bee Anatomy and Identification
Using any of the features listed below, identify which of the images are of bees. Remember: Bees can be distinguished from some mimics by looking for:
- Two pairs of wings (four total) — Flies only have one pair of wings
- Long uniform antennae
- Setae — wasps generally lack setae
If the image is not a bee, what type of insect might it be? If the image is of a bee, label each of the following parts if they are visible:
- Fore- and hindwings
- Compound eyes
- Scopa (if present)
- Head, thorax, and abdomen
Gullan, P. J., & Cranston, P. S. (2014). The Insects: An Outline of Entomology (Fifth Edit). West Sussex, UK: John Wiley & Sons, Ltd.