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Insects

Insects are the dominant class of arthropods in every habitat but salt water (where crustaceans reign supreme).

Over 950,000 currently living species have been identified, but the true number is surely much larger. As certain habitats are explored more thoroughly (notably the canopy of the tropical rain forest), more than half the insect species found turn out to be new to science.

The insects are classified into some two dozen orders on the basis of such differences as metamorphosis, wing structure, and mouth parts. This table lists 12 of the most prominent orders.

Major Insect Orders
Order	Examples
Orthoptera	grasshoppers
Hymenoptera	bees
Diptera	flies and mosquitoes
Lepidoptera	butterflies
Coleoptera	beetles
Homoptera	cicadas
Odonata	dragonflies
Hemiptera	true bugs
Siphonaptera	fleas
Anoplura	lice
Thysanura	silverfish
Blattodea	cockroaches and termites

The body of all insects is divided into 3 regions:

head. It is made up of several segments containing mouth parts and a segment containing a pair of antennae.
thorax, with three segments
- prothoracic segment. One pair of legs.
- mesothoracic segment. Another pair of legs and a pair of wings
- metathoracic segment. A third pair of legs and a pair of wings (halteres in diptera).
abdomen with several segments.

The insects are well-adapted to living on land, thanks to

an exoskeleton which is impervious to water and thus prevents loss of water from the body;
a system of tracheal tubes which penetrates every portion of the body, taking oxygen to, and carbon dioxide from, every cell;
paired, jointed appendages
- those in the head used for feeding. In the grasshopper (shown here), the mouth parts are adapted to cutting and chewing plant material.
- those in the thorax used for locomotion
wings (insects were the first animals to fly);
internal fertilization and embryonic development within a waterproof egg;
excretion of nitrogenous wastes as uric acid. Uric acid requires very little water for its excretion and what little it does need is reclaimed in the rectum. So insects deposit wastes that are practically dry.

Some Notable Insects

Drosophila. A "model" organism that has taught us much about genetics and development. Link to a discussion with further links to specific topics of Drosophila biology.
Mosquitoes. More than simply a nuisance, some are vectors of serious diseases such as malaria. [Link]
Honeybees (Apis mellifera). An example of a colonial insect (others are ants and termites). Of enormous economic importance, not for its honey, but for the role it plays in pollinating important agricultural crops.

The remainder of this page is devoted to honeybees.

The Honeybee Colony

The life of the honeybee colony revolves around the activities of its single queen.

When food is abundant she spends most of her time laying eggs in the wax cells of the honeycomb. The queen fertilizes most of these eggs, just before they are deposited in the cells, by releasing sperm from storage sacs (sperm receptacles), that were filled when she mated.

These eggs hatch into larvae (grubs) after 3 days.

The larvae are tended and fed by workers for 6 days — first with a protein-rich secretion called larval food, then with honey and a pollen/honey mixture called bee bread. At the end of this time, the workers cap the cells with wax, and the larvae undergo metamorphosis. Three weeks after the egg is laid, a new worker bee emerges. She is a female but does not have functional sex organs.

If the hive becomes overcrowded, the workers build several queen cells into each of which the queen deposits a fertilized egg. The grubs that hatch are fed larval food exclusively and for a longer period (right up to pupation). (The grubs destined to become workers get only a small amount of larval food for a brief period.) As a result, the grubs that hatch in the queen cells will develop into fertile females, one of which will become the future queen of the hive. Because of its crucial role in converting grubs into queens instead of workers, larval food is also known as "royal jelly".

The queen also lays unfertilized eggs that develop into male bees [see why], the drones. Drones are haploid.

So an active hive contains

one queen
a few hundred drones
many thousands of workers

Before a new queen emerges, the old queen leaves the hive, taking a substantial fraction of the workers with her. This is swarming.

After a few days, the new queen leaves the hive to mate (while flying high in the air) with the fastest of the drones chasing her (who gives up his life as he succeeds). She may repeat the process 10–30 times, thus ensuring a genetically diverse collection of sperm. This is important as colonies with a single father rather than multiple fathers

are less resistant to disease;
bring less food back to the hive;
build new comb more slowly and
raise fewer young.

After first stinging to death any other developing queens (unless the hive is so crowded that additional swarms are called for), she begins egg-laying.

The Work of the Hive

From the moment a worker emerges from her cell, her work for the colony begins. For the first 3 weeks of her life, she stays within or close to the hive. Some of her duties during this period (often in this order):

Cleaning cells for reuse.
Nursing the grubs. At first she feeds them brood food. When her glands stop producing this, she feeds them honey and bee bread instead.
The wax glands on the ventral side of the last four abdominal segments begin to secrete beeswax. The worker uses her mandibles to mold them into new cells for the comb.
Toward the end of her stay in the hive, her duties include
- receiving nectar, which the foragers bring back, and converting it into honey. This involves evaporating water from the nectar and digesting the sucrose into glucose and fructose.
- cooling the hive if it gets too hot. To do this, she receives water from foragers, spreads it on the surface of the combs, and evaporates it by fanning it with her wings.
- keeping the hive clean by removing any debris (incl. dead workers);
- patrolling in front of the hive, ready to sting any intruders.

Although all this sounds like the proverbial "busy bee", young workers actually spend only about 40% of their time on these activities. The rest of the time is spent simply standing around in the hive ready to engage in any of these activities as the need arises.

After three weeks in or near the hive, the workers take to the field as foragers of nectar and pollen. DNA chip analysis reveals that this shift coincides with a change in gene activity (mRNA levels in the brain up or down) of over 2000 different genes. One of these — whose activity goes up — is called Amfor ("Apis mellifera foraging gene"). [More]

Nectar is collected in a special chamber of the digestive tract, the honey sac, and brought back to the hive to be converted into honey. Pollen is brought back in the pollen baskets of the metathoracic (hind) legs. It is moistened with honey to form bee bread.

Tools of the Honeybee

Eyes

Honeybees, like all insects, have compound eyes. These give little information about depth but are very sensitive to "flicker effect".

It has long been known to bee keepers that honeybees respond better to flowers

moving in the breeze
with complex petals

The importance of flicker effect can be demonstrated by training honeybees to visit food placed on cards with patterns. For example, the bees can distinguish any figure in the top row from any figure in the bottom row more easily than they can distinguish between any of the figures in either row.

The honeybee eye can detect all colors (except red) and also ultraviolet. So the only red flowers that are normally pollinated by bees are those that give off ultraviolet as well.

Link to discussion of compound eyes.

Antennae

These contain odor receptors. Despite claims to the contrary, the bee's ability to detect and discriminate among odors is probably no better developed than ours.

Mouth Parts

These contain

the proboscis with which the bee feeds on liquid food, either the nectar of flowers or the honey stored within the hive. Her maxillae and labium are modified to form a tube through which the food is drawn up into the mouth by the pumping action of the tongue.
mandibles. Used to manipulate solids: e.g., pollen, wax, debris in the hive. (They can also be used as weapons.)
taste receptors. The bee seems to distinguish the same tastes we do [Link]. However,
- the bee's threshold to sour and salty is lower than ours while
- its threshold to bitter and sweet is higher. The relatively high threshold to sweet is of value to the bee as it prevents her from gathering nectar too dilute to be converted efficiently into honey.

Legs

The legs of the honeybee are far more than simply structures to walk on. They are highly specialized manipulative organs that enable the bee to carry out many of her tasks.

Prothoracic Legs

The have:

a pollen brush = hairs on the first tarsal segment that collect pollen from the front of the body.
an antenna cleaner = a notch in the first tarsal segment in which the bee can place her antenna. Then when she crooks her leg, a spur on the tibia holds the antenna in place while she cleans off pollen, etc. in a wiping motion.

Mesothoracic Legs

They have:

pollen brushes on the first tarsal segments. These scrape pollen off the thorax and also from the pollen brushes on the prothoracic legs.
a spine that sticks down from the tibia. It may be used to dig plates of wax out of the wax glands.

Metathoracic Legs

Each has a:

pollen brush on the inner surface of the first tarsal segment. This pollen brush collects pollen from the pollen brushes on the mesothoracic legs.
pollen packer (also called a pollen press) in the joint between the tibia and the first tarsal segment. With the stiff spines (the rake) of the pollen packer on one leg, the bee removes the pollen from the pollen brush of the opposite leg. Straightening the leg then forces the anvil-shaped auricle up against the pollen caught under the rake. The pollen is squeezed through the joint and up into the
pollen basket on the outer surface of the tibia.

All these manipulations are carried out in the brief interval it takes the bee to fly from one flower to another.

Foraging

Foraging bees can communicate to each other information about the odor, richness, distance, and direction of food. Link to a discussion of how this is done.

How is one to follow the behavior of a single honeybee as hundreds of bees enter and leave the hive? This photo (courtesy of Dr. M. Renner) shows how a drop of paint is applied to a foraging bee so that she can be identified on her return to the hive.

Sex Determination in Honeybees

In honeybees, the default sex is male.

In order to make females, the zygote must have received

two copies of the csd ("complementary sex determiner") gene AND
these must be different alleles.

Eggs that are not fertilized contain only one of the queen's csd alleles and develop into males. In the unlikely event that a fertilized egg receives two identical alleles, a diploid male is formed (and usually eaten).

Presumably, the proteins produced by csd must be different to make a female. They usually are, because the csd locus is polymorphic — with some 19 alleles ("multiple alleles") discovered so far in the species.

This method of sex determination is quite similar to the methods that many plants use to prevent inbreeding. Link to a discussion of self-incompatibility systems in plants.

Queens vs. Workers

The anatomy of queens and workers differs in many ways (e.g., only workers have a stinger, pollen packer, etc.), but their genomes do not. Clearly the development of female larvae down one path or the other requires a different coordinated genetic program for each. But how can diet dictate which pattern of gene expression to launch? The answer appears to be epigenetic changes to the genome induced by one or more components in royal jelly.

Link to a discussion of epigenetics.

In the 28 March 2008 issue of Science, Kucharski, R. and colleagues reported on experiments that revealed DNA methylation patterns distinguishing the development of workers and queens.

They fed female honeybee larvae on a semi-synthetic diet that did not contain enough royal jelly to produce queens. And, indeed, untreated control larvae produced 238 workers, no queens (but 73 intermediate "queen-like" adults).

However, larvae fed on the same diet that were injected with a small interfering RNA (siRNA) that silenced a gene that methylates cytosines (C) in the genome produced 188 queens and only 74 workers (and no intermediates).

So differential methylation patterns in DNA appear to determine whether a female larva develops into a worker or a queen.

Link to a discussion of how antisense RNA is used in the laboratory to silence the expression of single genes.

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16 September 2018