We learned in an earlier part of the course that water loss by transpiration across the integument of insects is restricted by cuticular hydrocarbons. Restriction of water loss is probably the principle, but certainly not the sole, biological role of hydrocarbons. Hydrocarbons also serve as semiochemicals. Semio is root of the word signal, and semiochemicals serve as chemical signals. Some of the signals that insects send and receive include sex attractants, aphrodisiacs, anti-aphrodisiacs, kairomones, aggregation pheromones, and recognition cues for species, caste, and kin. Leaders in pheromone research speculate that many other semiochemical roles of hydrocarbons remain to be discovered.

As usual, you will not be surprised to learn that the chemistry, biochemistry and biology of hydrocarbons, hydrocarbon-derived pheromones, and fatty acid-derived pheromones could easily occupy a semester's course. Just to make the point, let us consider a few examples of cuticular lipids which function as semiochemicals in insects.

We would not want to exhaustively list the hundreds of chemical now known to serve some function in chemical communication. Instead, let us turn attention to a few specific pheromones.

The sex pheromone of the housefly, Musca domestica, has four components. They are (Z)-9-tricosene, cis-9,10-eposytricosane, (Z)-14-tricosen-10-one, and methylalkanes. These components are produced by females, and they influence male courtship behavior. Z9-23:Hy (the Hy represents hydrocarbon) increases male mating strikes toward females and toward males. Z9-23:Hy has the trivial name muscalure. The epoxide and the ketone decrease the number of homosexual mating strikes in the presence of muscalure. Please take the point that these two components are sex recognition cues. The final component, the methylalkanes, serve as an arrestant, and they increase the length of time males spend with females, and with chemically-treated inanimate models.

The synthesis of these pheromone components is related to the age of females. Newly emerged females do not have detectable amounts of pheromone, and sexually mature females have large amounts. Pheromone biosynthesis was related to ovarian development by monitoring pheromone synthesis during oogenesis. Pheromone synthesis did not begin until the ovaries reached stage 4, the early vitellogenic stage. This is also consistent with reproductive behavior: females with previtellogenic ovaries do not attract males, and females with stage 4 through stage 10 ovaries do attract males.

Based on general endocrine principles, sex pheromone production in female houseflies could depend on JH, which influences ovarian development. In some cockroaches, for example, JH induces vitellogenesis and sex pheromone biosynthesis. Alternatively, house fly sex pheromone production could depend on another factor produced by stage 4 and older ovaries. These possibilities (i.e., sex pheromone production stimulated by JH or by an ovarian factor) were examined by observing the effects of removing the CA-CC complex in some females and removing the ovaries in other females within a few hours of emergence. The females with the CA-CC complex removed produced the pheromone components. On the other hand, the ovariectomized females did not produce the pheromone. Terry Adams and his colleagues concluded that a factor from the ovaries was responsible for pheromone production.

Because mature ovaries in certain Diptera produce ecdysteroids, it was reasonable to suppose that ecdysteroids stimulate pheromone production. This was tested by overiectomizing newly emerged females, then treating them with 20-hydroxyecdysteroid. By 18 hours after hormone treatment, the ovariectomized females produced pheromone. These results would support the idea that 20-hydroxyecdysteroid regulates sex pheromone production. Let us recall, however, a common thread in scientific research: a single experimental result is not persuasive. Adams and his colleagues conducted additional experiments on the issue. For one, JH treated females did not produce pheromone, which showed that pheromone production results from a specific hormone. For another, adult male houseflies were treated with 20-HE, after which the males produced the female-specific pheromone. This experiment shows that males and females are able to synthesize pheromone, and that pheromone synthesis is induced by 20-HE. Finally, isolated male abdomens could be induced to produce pheromone by 20-HE treatments. This result shows that factors from the head, such as neurosecretory hormones, are not involved in pheromone biosynthesis. Consider an outline of pheromone biosynthesis in houseflies. The point to take from is that an ordinary fatty acid, 18:1, is elongated from 18:1 up to 28:1 by simple addition of acetate units. An intermediate in this pathway, 24:1 is decarboxylated into a hydrocarbon one carbon shorter than the fatty acid precursor. Also note that males and sexually immature females produce 27 carbon hydrocarbons. You can see that the enzymes that decarboxylate the 24:1 intermediate are the controlling steps in pheromone production.

The sex pheromone of the German cockroach is a contact pheromone produced by females and perceived by males through antennal contact. The pheromone is made of four components, each of which produces the complete program of mating behavior in males. The pheromone components include 3,11-dimethylnonacosan-2-one, 20-hydroxy-3,11-dimethylnonacosan-2-one, 29-oxo-3,11-dimethylnonacosan-2-one, and 3,11-dimethylheptacosan-2-one.

Biosynthesis of the pheromone components looks to be a bit more complicated, but we can see through the complications. Ordinary fatty acid synthesis looks something like the attending figure. Figures such as this one are depicted in virtually every biochemistry textbook on earth. Fatty acid biosynthesis is simply a repeating cycle of condensation, reduction, dehydration and reduction. We can turn to pheromone biosynthesis, and see that the pattern is similar, with the exception that the methyl branches are incorporated into the acyl chain by insertion of a methylmalonyl-CoA unit in place of a malonyl-CoA unit:.

There is a theme in pheromone biosynthesis, which we can see in the cockroach system. The dimethyl fatty acyl is decarboxylated to a hydrocarbon, which undergoes further metabolism to the alcohol and then to the ketone.

Sex pheromone production is under endocrine regulation in the German cockroach. Here, JH stimulates ovarian development and stimulates sex pheromone biosynthesis. The picture is complicated by the point that biosynthesis of the hydrocarbon substrate appears to be controlled by feeding. Then JH controls the conversion of hydrocarbon to pheromone.

Sexually mature females of many lepidopteran species exhibit calling behavior, which takes the form of everting their pheromone glands and allowing pheromone to evaporate into the air. Males respond to the pheromones by flying in a very poorly defined upwind direction, toward the source of the pheromone. Lepidopteran sex pheromones are usually blends of acetate esters of fatty acids.

In the cabbage looper, regulation of sex pheromone biosynthesis differs from the housefly and German cockroach. In the cabbage looper, pheromone glands are competent to synthesize pheromone at adult eclosion. The competence is controlled by ecdysone during the pupal stage. In this insect, it is the release, not the biosynthesis, that is regulated.

PBAN is an acronym for pheromone biosynthesis activating neuropeptide. The idea that a brain factor was necessary for pheromone biosynthesis goes back about 10 or 11 years. More recently, about 4 or 5 years ago, the peptide was purified and sequenced. PBAN is composed of about 33 amino acids, and has been purified from a number of insect species.

PBAN is produced in the subesophageal ganglion. There used to be some controversy on the point, but a number of groups express the idea that PBAN is transported from the SOG to the CC. The hormone is released from the CC into the hemolymph, and finally interacts with specific receptors in pheromone gland cells. It is thought that the hormone activates a calcium channel, which causes increased intracellular calcium. The calcium may exert several actions, all of which lead to activation of the pheromone biosynthesis pathway. This mechanism probably occurs in various species of Heliothis and other moths, as well.

Once again, insect physiology is a study in comparative physiology. The details of regulating pheromone release are just a bit different in the red-banded leaf roller. In this moth, PBAN interacts with receptors in the corpus bursae, which in turn releases a bursa peptide that ultimately interacts with cells in the pheromone gland.