The Mysteries of the Metapleuron

I've always found Nysius to be a difficult genus to identify, apart from the obviously hairy N. huttoni. I turned up a few at the weekend but this time I used the key published by by Dolling, Het News, Issue 2, Autumn 2003. This turned out to be much easier than the other keys, and in the blink of an eye I had this one down to a species pair - N. graminicola vs. N. senecionis. The hind tarsi indicated which of these two it was, but to confirm I had to check the metapleuron, which confirmed that this was Nysius senecionis. 

Which raises the question - what the heck is a metapleuron? In Nysius the metapleuron is the lateral sclerite of the third thoracic segment and is the primary platform for the metathoracic scent gland (MTG) system. It consists of:

• The ostiole: an opening on the metapleuron through which the MTG reservoir empties its contents.
• The peritreme: a specialized cuticular structure (often shaped like an auricle or spout) that surrounds the ostiole. This directs the liquid secretion away from the body and onto the evaporatorium.
• The evaporatorium (or matt area): a distinct, non-glossy region of the metapleuron characterized by a highly micro-sculptured, "mushroom-like" or reticulated cuticle which increases the surface area significantly, allowing the defensive fluid to spread rapidly and evaporate (volatilize) almost instantly. This creates a "scent cloud" that deters predators before they can make physical contact.

The most important physiological role of the metapleuron is the dispersal of repellent defence chemicals. An additional role is in pheromone communication, acting as a broadcast structure for alarm, aggregation and sex pheromones. The specialized cuticle of the metapleuron evaporative area also prevents defensive secretions from being absorbed back into the insect's own underlying tissues.

I don't know about you, but it seems like a metapleuron might come in quite useful.