Re: If a butterfly's wing is torn, will it grow back?

No.  Generally insects (except for at least some insects with simple 
metamorphosis, which can regenerate legs under certain circumstances) do 
not regenerate any body parts, although most spiders can regenerate legs 
before they reach adulthood.   This is an interesting question, because 
some organisms can regenerate limbs or sometimes other body parts.  
Insects with complete metamorphosis (those with egg, larva, pupa and 
adult) and with wings are always adults in any case, and adults of 
arthropods are unable to regenerate body parts because they do not molt 
again.  Only the ability to molt would allow them to regenerate. Insects 
only have one set of wings (two pairs in most, but only one pair in true 
flies.)  Once one of the wings is damaged or lost the insect will not be 
able to fly well, or possibly not at all. 

I have no good reference on this exact issue, but you should try:

Indian stick insect (Stick insect leg regeneration)  http://centralpets.com/pages/critterpages/insects/stick_insects/STK4320.sht
ml

Insects and Spiders of New Zealand (Spider leg regeneration)  http://www.landcareresearch.co.nz/education/insects_spiders/spiders.html#re
gen