How to Identify Dytiscids(diving beetles) from Hydrophilids(water scavenger beetles)!

  

Just by looking at these two images, can you tell what family each one belongs to? These two insects look quite similar, but if you know what to look for, you shall be able to tell which one is which. The first image is that of a Dytiscid(predacious diving beetle), and the second a Hydrophilid(water scavenger beetle). Here is how to tell them apart by appearance: 
Dytiscids have a rounded ventral(under) side, while Hydrophilids have flat ones. Flipping the insects totally over onto the ventral sides, you may notice that Hydrophilids have an elongated spine stretching across their body segements, while Dytiscids lack this. Here is an image of what their ventral sides look like:  
 Hydrophilid spine

Dytiscid ventral side

These are the main physical differences between the two, but there are other differences in their diet. Hydrophilids are scavengers, while Dytiscids are predators. However, Hydrophilids will occasionally prey on mosquito larvae. Dytiscids also dive to capture prey, hence the name predacious diving beetle. When ready to dive, the Dytiscid stores air under its wings to breathe while it is underwater. In order to be so agile in the water, Dytiscids are very well streamlined, in fact, so streamlined that males need to have suctions on their legs to grip females while mating!