adony melathopoulos wrote
> Will someone explain to me why resistance to oils cannot develop ?  If I
> understand the mechinism by which mineral oil is hypothesised to work (and
> excuse me if I miss important details), mites die because their spiricles
> get clogged up with oil.
 
Theoretically mites could develop resistance to oil if they could evolve
spiracles wich do not clog with oil. Why not considering the vastness of
the universe and it's diversity. But it would be like humans tryng to
acquire the capability to breathe underwater by drowning people. The
suceptibility to oil is not comparable to the suceptibility to
physiologically active substances. In the case of the oil, the principle
involved depends on two factors: small spiracle size and the high
surface tension of oil which permits it to spread and form uninterrupted
airtight film. Also, insect spiracle are generally bordered with
hydrophobe hair that protect against foreign particules and water but
that are oleophilac (not sure of the english term but it means "oil
compatible").  In the case of physiologicaly active substances the
principle of resistance is based on the capacity to neutralize nocives
or deadly molecules by degradation or elimination. This capacity vary
from one individual to another and involves substances such as Mixed
Function Oxidase (MFO), cytochrome P-450 and others which exist in most
of all living animals. As the individuals who are the most performant
have the most chance to breed and produce offspring carying their genes,
an environnemental pressure such as intensive use of pesticide will
cause the evolution of the specie into more resistance to the stressfull
agent. Specially in the case of insects wich reproduce at a tremendous
rate.