5. Function of brochosomes
 
The function of brochosomes remains poorly understood. Several hypothetical functions have been proposed, but most of them have not yet been tested experimentally (however see Velema et al. 2005 on the role of egg brochosome coats as protection against egg parasitoids).
 
Not simply waste products. The Malpighian tubules are primary excretory organs of insects but, in leafhoppers, this appears to be the function of only thin parts of the tubules, while the inflated brochosome-manufacturing segments are entirely involved in synthesis. Similar cases of secondary synthetic specializations of the Malpighian tubules are known in various insects. For example, these organs manufacture products such as silks or other components of cocoons (Neuroptera, Rhaphidioptera, some Hymenoptera), defensive secreta (some Neuroptera), components of scale covers (Hemiptera: Diaspididae), substances that agglutinate soil particles (immature Cicadidae) or foam the "spittle" produced by immature spittlebugs (Hemiptera: Cercopoidea).
 
 Hypothetical functions  Likelihood*
 Repellence of water and sticky substances (e.g., excrement)  Very likely (esp. integumental brochosomes)
 Protection against fungal or microbial pathogens  Likely
 Protection against parasites  Likely (esp. egg brochosomes**)
 Protection against desiccation Likely in some cases
 Thermoregulation, Protection against UV light Likely in some cases
 Pheromone reservoirs Unlikely
 
*See Rakitov 2002b for detailed discussion.
**See Velema et al. 2005 for experimental evidence.
 
 
 
 
Multiple functions possible. As a rule, body-coating structures (feathers, mammalian hairs, lepidopteran scales, etc. ) have multiple functions. Brochosomes, which form coats on the integument of leafhoppers, have probably evolved multiple functions as well.
 


Above: Brochosomal coatings can be remarkably regular, both dense and thin. This one, on the lower surface of a forewing, is practically a monolayer.
 
Water-repellent properties. Water and water-based liquids do not adhere to brochosome-coated bodies of leafhoppers. The reason for this is a very complex fractal surface geometry formed by the layer of brochosomes. In other animals and plants, the extreme water-repellence is achieved by either development of a surface microsculpture, or secretion of microscopic wax particles. It is an important property for leafhoppers because it protects them from 1) their own copious liquid excretion (filtered plant sap), which is often sticky, 2) rain and dew, and possibly 3) spider webs.
Above: Droplets of water adhere to the integument of a leafhopper specimen (Nionia palmeri (Van Duzee)) poorly coated with brochosomes (A), but are repelled by the thick orange brochosome coat of another specimen (B). Variation in the development of the brochosome coat in this species is natural.
Above: Droplets of water on a smooth hydrophilic (A), smooth hydrophobic (B), and rough hydrophobic (C) surfaces. In the last example, the roughness of the surface is formed by a layer of microparcticles, such, for example, as brochosomes.
 
Brochosomes as an alternative to wax. In many other insects, the integument produces tiny chunks of fibrils of wax, forming a water-repellent coat. Although such particulate waxes differ from brochosomes in both chemical nature and origin, the functions of these two biomaterials in many cases appear to be similar. Waxy particulate coats are especially common and diverse among the plant-feeding Hemiptera, other than leafhoppers ­ planthoppers, aphids, whiteflies, and scale insects. Just as leafhoppers, these hemipterans need protection from their sticky sap-derived excretion, as well as from a variety of other hazards. In several such groups, the integumental wax is also used as a coat of the eggs. Unfortunately, as in the case of brochosomes, the properties and the adaptive significance of the particulate waxes have never been studied experimentally and, therefore, remain hypothetical.
 
Read more: Rakitov 2002 b, Velema et al. 2005
 
Back to Contents