A Quick Lesson in Larval Identification and An Identification Key
Identifying a larval form in the field can give you a "first cut" at the identification of the critter chewing the leaf in front of you. Identification to species often requires source books and detailed keys, but identification to order is normally first step to using these references. Often some basic identification to order is all you really need to know. Try this for a quick identification assist. It will not work with "everything", but should handle most of the common "stuff".
1. Larva without any legs either on the abdomen or the thorax. No distinct head. Body tapering to a "sharp" point at one end. Maggots (Order Diptera = flies)
1'. Larva without legs, but with a distinct head. Body tapering to a "blunt" point.
Grubs (Order Coleoptera = beetles)
1". Larva with legs. Legs may be true segmented legs on the thorax or fleshy appendages (these are "prolegs" and may be easy to discern or just indistinct blobs of flesh used for walking) on the abdomen - 2
2. Only legs present are true legs with segments. These are on the thorax. - 3
2'. Abdominal prolegs on abdomen along with true, segmented legs on the thorax - 4
3. Body caterpillar like, insects freely walk on foliage
Leaf beetles and Chrysomelid beetles (Order Coleoptera = beetles).
(Leaf beetles may be pests and need suppression actions, lady bugs are beneficial predators of aphids and should be protected)
3'. Body plump and C-shaped. Soil dwelling
White Grubs, specifically, Scarabid beetles (Order Coleoptera= beetles).
4. Only one pair of abdominal prolegs present at the far end of the abdomen. Leaf beetles: (Order Coleoptera)
4'. More than a single pair of abdominal prolegs present - 5
5'. 6 to 8 pairs of abdominal prolegs present
Sawfly (Order Hymenoptera = Bees, wasps and sawflies)
(Sawflies can not be killed with BT sprays)
5'. 2 to 5 pairs of abdominal prolegs present
Caterpillar (Order Lepidoptera = Butterflies and moths) - 6
(Caterpillars can be killed with BT sprays)
6. 2 pairs of abdominal prolegs
Looper (specifically, Geometrid caterpillars: Order Lepidoptera)
6'. 3-4 pairs of abdominal prolegs
False looper (specifically, Noctuid caterpillars Order Lepidoptera)
6". 5 pairs of abdominal prolegs
Caterpillar (remainder of Lepidoptera)