You will need to kill the live insects you capture before putting them into your collection. The killing method should be quick and as painless as possible. Also, the killing method should not ruin the insect's appearance. A killing jar that can be carried with you is handy for doing this. You should prepare one before going out to collect.
Any clear, wide-mouthed, plastic jar with a tight screw-cap lid can be used to make a killing jar. A pint-size jar (a peanut butter jar works well) is easy to carry and will be big enough to hold large insects. You can make larger or smaller killing jars to suit yourself.
After selecting a jar, cut discs of blotter or newspaper to fit snugly into the bottom of the jar. A stack of discs 1/2-inch high is enough.
When you want to use the killing jar, pour fingernail polish remover or rubbing alcohol onto the paper discs. Pour off any fluid that is not absorbed by the paper. Then put a few narrow strips of tissue paper in the jar. The tissue paper helps absorb moisture and gives the insects a place to hide. When insects are hiding, they do not thrash around and damage themselves so much.
Killing jars containing cyanide, carbon tetrachloride, gasoline, or ether are too flammable or poisonous to be used safely. Even though fingernail polish remover is relatively safe, the killing jar should be labeled KILLING JAR--POISON.
If you are near home, you can collect insects in a plain jar and then put the jar in the freezer to kill the insects. They will be knocked out after a few minutes, but keep them in the freezer for at least an hour. If you take them out too soon they may revive after you have pinned them.
Some insects, like beetles and grasshoppers, can be drowned in four or five seconds in warm soapy water without spoiling their appearance. This is not a good killing method for hairy or scaly insects or small, delicate ones because they will still look soggy even after they have dried.