Well, They're alike because the animal changes in form from juvenile to adult, with different habits (place it lives (water/land), diet, etc) and even different,organs (gills in amphibian larvae, wings in adult insects...). They're different because they evolved separately and the mechanisms are very different.Amphibians morph continuously, losing their gills and gaining their legs gradually. Insects molt, and every time the animal emerges it's changed with a jolt. Pupating insects change even more abruptly, going dormant for weeks and emerging totally different just like that. And where adult frogs may inhabit the same pond as their tadpoles, insects will take to the air and take up totally different ways of life. Some adult insects don't even have a mouth.