Family is Really Praying Mantis's

by Deluth
(Grand Junction, Colorado, USA)

I watched a movie when I was young where a family is really giant bugs (praying mantis's) from another planet.

At one point, the dad (whom I thought was John Lithgow) is looking at porn magazine but it's really a National Geographic article about mantis mating.

The daughter gets pregnant and gives birth to an egg sac, and her boyfriend promptly freaks out, and steps on it.

Not knowing this movie has driven me crazy for years. Does anyone have any ideas?