A butterfly has 2 pairs of wings, 3 pairs of legs, and 1 pair of antennas. There are 10 groups of 13 butterflies. How many less antennas than wings and legs combined?
Since each butterfly has 2 pairs of wings, it means there are 2 x 10 = <<2*10=20>>20 pairs of wings.
The number of wings is 20 x 2 = <<20*2=40>>40.
Each butterfly has 1 pair of antennas, which means there are 10 x 2 = <<10*2=20>>20 pairs of antennas.
The number of antennas is 1 x 20 = <<1*20=20>>20.
Together, the wings and legs have 40 + 30 = <<40+30=70>>70.
So the butterflies have 70 - 20 = <<70-20=50>>50 less antennas than the wings and legs combined. Answer: \boxed{50}.