Extreme sexual dimorphism in scale insects: why do males and females look so different?

The amateur of gardening would have encountered those annoying and strange looking insects, covered with white cotton or a very hard small scale. Scale insects suck the sap of your favorite rose plant or cherry tree. But, blame it on the females! The insect you will encounter on the stems or leaves are always adult females or juveniles. The adult male is not only not easily spottable but when so, they just look so different that you might think it is a fly.

Scale insects are extremely sexually dimorphic. This dimorphism has been present for as long as scale insects originated, for the fossil record have not uncovered yet any female that look like males. To give you an idea of the dramatic dissimilarity, here are some treasures that I found from Flickr.

Ortheziidae (ensign scale insect) -> here

Pseudococcidae (mealybug) -> here (the male is on top of the female, while an ant is looking for honeydew on the female’s butt)

Coccidae (soft scale insect) -> here (the male is sitting on top of the brown flat female).

How does it happen?

Notebook 4-7

After hatching, males and females look just like one another. During the first instar nymph, even microscopic differences cannot be observed. However, at some point during the second instar nymph, males and females reveal their true selves. From then on, they undergo completely different developmental pathways.

  • The juvenile female carries on to successive molts, basically becoming bigger and bigger but not changing at all in appearance. By the last molt, the female is considered adult because of her reproductive organs, but will just look like a juvenile. Females are neotenic.
  • The juvenile male starts secreting white cottony filaments that will be used as protection and hide him away during his dramatic metamorphosis. Males go through non feeding stages where wings develop progressively. When the adult male emerges, he looks completely different from the juvenile, with wings and elongate body and long antennae and legs, but lacks mouthparts.

Scale insects belong to the order Hemiptera, which are hemimetabolous insects (developing through successive molts and progressive wing development). However, in the case of scale insects, both males and females act differently from their “normally” developing cousins: females don’t develop wings (never ever) and are virtually juveniles with reproductive organs, and males go through a development stage that looks like a fly metamorphosis (but not quite the same).

In short: scale insects are unconventional and developmental rebels.

My postdoctoral work is looking at the hormonal regulation perspective to understand how males and females, looking so similar in the beginning, followed different paths and ended up looking like different insects.

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