These insects belong to the Orthoptera, the same order as grasshoppers. Katydids are among the most abundant large insects in the rain forest, whereas grasshoppers seem rather rare. For the unpracticed, katydids are quite difficult to see as most species are cryptically coloredthey blend in with the background. Many species are colored green, and look like leaves. Katydids are more active at night than during the day, presumably to avoid predators.
You can tell this katydid is a female, from the long ovipositor protruding from the rear of the body. The ovipositor is a tube down which eggs pass in order to be laid safely deep in a layer of leaf litter. The young katydids are miniature replicas of adults, only lacking the wings.
This close-up of a katydid shows how it is similar to grasshoppers, and the difference: kaydid legs and antennae are usually longer than in grasshoppers. Looking into the face of such a creature shows we don't need to go into outer space to encounter aliens!