Soft yet Scaly (Taxon of the Week: Coccidae)

The stellate scale Vinsonia stellifera (Coccidae). Scales are insects that have abandoned motility for most of their lives to become sedentary plant suckers. Photo from here.

The truly bizarre insects known as scales have been covered at this site previously, including a brief description of the scale life cycle. In that post I referred to the ensign scales or Ortheziidae; in this post I'll cover the soft scales or Coccidae. The Coccidae include about 1000 species, some of which produce a dorsal covering of wax while others lack a dorsal covering (Williams, 1991). While ortheziids belong to the group of scale families known as archaeococcids, coccids belong to the more derived grouping known as neococcids. Neococcids are distinguished from archaeococcids by the absence of spiracles on the abdomen, and of compound eyes in the adult males (instead, male neococcid eyes have become reduced to dissociated ocelli). Coccids are distinguished from other neococcid families by the presence of a pair of rounded or triangular plates at the base of the anal cleft (Williams, 1991).


While female scales remain immotile for the rest of their lives once they have found a host, males regrow their legs and usually develop wings at maturity to find females. This is the Kuno scale Eulecanium kunoense. Photo by Joyce Gross (and very impressive it is too - photographing something as minute as a male scale would not be an easy call.

Another distinctive feature of neococcids is something referred to as Paternal Genome Loss (PGL - also known as Paternal Genome Elimination). In most neococcid families, males are technically diploid but early in development the chromosomes a male has inherited from its father are all inactivated so that it becomes functionally haploid. When the male produces sperm, these inactivated chromosomes are eliminated from sperm production and only the maternally-inherited chromosomes are passed on to its offspring. The reason for the evolution of PGL remains unknown*, but it appears likely to have evolved among neococcids on a single occasion (Yokogawa & Yahara, 2009). True haplodiploidy as found in Hymenoptera, where males are truly haploid as opposed to functionally haploid, has also evolved in scales of the archaeococcid family Margarodidae but is as yet unknown among neococcids despite suggestions that PGL may be a precursor to the origin of haplodiploidy. It is worth noting that, while an origin of haplodiploidy from PGL may seem reasonably intuitive, there is the small problem that there are more than twice as many known cases of taxa evolving haplodiploidy as PGL.

*Endosymbiotic bacteria such as Wolbachia have been shown to cause PGL in some insects (and the presence or absence of endosymbionts has been shown to affect PGL in at least one neococcid); alternatively, it could result from genetic factors on the animal's own X chromosome promoting the transmission of maternal chromosomes.

REFERENCES

Williams, D. J. 1991. Superfamily Coccoidea. In The Insects of Australia, 2nd ed. vol. I pp. 457-464. Melbourne University Press.

Yokogawa, T., & T. Yahara. 2009. Mitochondrial phylogeny certified PGL (Paternal Genome Loss) is of single origin and haplodiploidy sensu stricto (arrhenotoky) did not evolve from PGL in the scale insects (Hemiptera: Coccoidea). Genes Genet. Syst. 84: 57-66.