All Pachypodium are succulent plants that exhibit, to varying degrees, the morphological characteristics of pachycaule trunks and spinescence. These are the most general features of the genus and can be considered distinguishing characteristics. The pachycaule trunk is a morphologically enlarged trunk that stores water so as to survive seasonal drought or intermittent periods of root desiccation in exposed, dry, and rocky conditions. Whereas there is great variation in the habit of the plant body, all Pachypodium exhibit pachycaul growth. Variation in habit can range from dwarf flattened plants to bottle shaped shrubs to dendroid-shaped trees.
THis is the plant on the RIGHT. These are two different species of Madagascar Palm. The second picture shows a moth using the thorns on the trunk as a safe place to hang out.