Tachinid flies develop as parasites inside other insects. Tachinids are about the size of a house fly, generally gray or brown, and covered with dark bristles. They are rarely seen but often leave their 'calling card', a white egg laid on various caterpillars, beetles and bugs, usually near the head. Douglas-fir tussock moth, tent caterpillars, and fall webworm are among the insects commonly attacked by tachinid flies.
The eggs hatch within the day and the young fly maggots tunnel into their host. There they feed for about a week (carefully avoiding the vital organs until the end), eventually killing the host insect.
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