More on Pollination – Pollen Sac

Well, I made my presentation and as you might guess there were a number of questions I couldn’t answer. One was how does a bee’s pollen sac hold pollen? Hm-m-m. That’s a good question.

Why do bees collect pollen?

To make  bee bread, of course. Pollen contains protein and fats — nutritious substances bees need to raise developing larvae and to live their adult lives successfully. Bees harvest pollen from a variety of blooming plants to help provide the most complete nutrition for the baby bees and the healthiest colony. Pollen is also important for the development of the queen bee’s reproductive process.

Nectar and honey are an energy source. But this carbohydrate source alone will not provide adequate winter food for honey bees or raise strong young bees without some pollen. So the pollen and nectar are mixed with water which causes the pollen granules to grow and form a kind of bread. The bread is then tucked away in a honey comb for future use by the larvae.

Bee Bodies

Bumble bees and honey bees are very fuzzy animals so pollen sticks to them as they visit flowers collecting nectar. It sticks to their antennae, their legs, their faces, their bodies — everywhere – which enables them to then share with the plants they visit or at some point, clean from their bodies.

Evolution has provided a special structure located on their front legs just for cleaning their antennae. It looks like a notch in their leg. The inside curve of this notch is lined with hairs that work like a comb. Bees will draw their antennae through this notch, so the hairs comb off the pollen.

Their middle legs are also equipped with comb-like hairs, which they use to scrape pollen from their body.

What is a pollen sac?

Male bees do not actively collect pollen, only the queen and worker bumblebees do. They transfer the pollen they collect to the sacs or baskets on their hind legs to make it easier to transport back to the hive. Bumblebee pollen sacs or baskets are known as corbicula.

Bees have a tibia (lower leg) just like humans. The tibea has a flat, somewhat convex and shiny spot on it that is surrounded by hairs. Some of the hairs are longer and stiff, and they form the pollen sac. Located at the lower end of the tibia is another comb-like structure which works with the foot and ankle to press the pollen into the sac. The female bee will push the pollen gathered on her body down toward her lower legs, where she mixes the pollen with a little nectar and then packs it into her pollen basket from the bottom up. The hairs in the pollen basket hold it in place. When the pollen sac is full, it can hold more than one million grains of pollen. 

Not all bees have sacs; they simply collect pollen on their hairy bodies. Orchard mason bees and leafcutter bees are good examples. Once they get back to their nest, they clean up and stuff the pollen around each egg.

Other bees have a similar apparatus, only it is called a scopa. This basket can be located on the hind legs and/or the ventral side of their abdomen. Honeybees have crevices on the backs of their knees.

How does the pollen move about?

As mentioned earlier in my Pollination Continued post, pollen can be blown in the wind or it can be carried by an insect, bird or animal to a willing ovule. Both of these methods are a bit haphazard, so how does the pollen ensure that it finds its target plant?

The surface features of grains of pollen help them hitch a ride with visiting insects, animals and birds. Pollen can be dotted with little spikes, like Velcro, or have weblike surfaces. Others are enshrined in thread-like tangles or have dimples or ribs. Some pollen sails through the air on wings.

Plants, on the other hand, often lend a hand by providing sticky or feathery parts to ensure and capture the pollen.

Through evolution, pollinators and plants have learned to make the pollination process as efficient as possible. Flowers have evolved to develop the specific color, fragrance and/or shape to make themselves attractive to nectaring pollinators, and pollinators have reciprocated by developing specific body parts to enable them to pass pollen along inadvertently as they go about their daily lives.