Q:

How do you refer to a member of a biological taxon in the singular? 

If I wish to refer to, say, an Owl in the more generic form as a member of the biological order Strigiformes, what is the correct modification of the apparently plural word into a singular form? 
For instance:

Upon entering the forest, Jonothan was immediately assaulted by an angry [singular strigiformes]. 

Or:

The Manduca Sexta is a [singular lepidoptera] which feeds on a [singular solanaceae] during the larval phase, normally a Tobacco plant. 

A:

It depends:

One of the strigiformes is a strigiform.   Or an owl. 
One of the lepidoptera is a lepidopter:

1881 Elwes tr.  De S. Pinto’s How I crossed Afr.  I. v. 120 ― This gigantic lepidopter, when young, feeds upon the grasses. 

One of the solanaceae is a solanacean.    Or a nightshade.