Movement of Insecticides through the Cuticle

It is clear from the preceding discussion that insect cuticle can be considered a two-phase, lipophilic-hydrophilic system. The outermost phase is waxy and hence hydrophobic (i.e., lipophilic). Because most insecticides are nonpolar, this first barrier is advantageous to their contact action. Therefore, in insects, the contact toxicity of an insecticide is similar to the oral toxicity. In contrast, the acute oral toxicity is much higher in mammals than the contact toxicity because mammalian skin is relatively resistant to the entry of insecticides. 

Source From Lovell, J.B., /. Earn. Entomol., 56, 310,1963. With permission. 

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