Please check these links.

https://www.researchgate.net/publication/233555426_A_Simple_Penetrometer_for_Measuring_Leaf_Toughness_in_Insect_Feeding_Studies

https://www.researchgate.net/publication/226421526_Leaf_Toughness

https://archive.org/stream/cbarchive_48847_aportablepenetrometerformeasur1884/aportablepenetrometerformeasur1884_djvu.txt

Article A Simple Penetrometer for Measuring Leaf Toughness in Insect...

Chapter Leaf Toughness

Thanks a lot!

Hi Elvira. There is no one instrument because leaf toughness is a composite of many factors; see Sanson G (2001) Austral Ecology 26: 535-546. Most entomologists do not think much of penetrometers. You need to decide what leaf property or insect variable you actually want to measure and balance that against your budget. Many people settle for specific leaf weight or leaf mass area since the equipment needed is multifunctional and the measurements speak to aspects of leaf physiology. If you really want measurements related to insect biting/chewing, you need to get into cutting equipment as outlined in Sanson et al. Martin.

Hi Martin, thanks a lot for your detailed reply. The paper is really good. We're interested in possible factors that could explain preference of Lygus bug and spider mites for one plant over another. Indeed it's hard to find a single device to measure "toughness", and I'm not quite sure how much information it'll give us, but I'd like to give it a try. 

I doubt whether any physical measure would say much about how a true bug gets its stylets into a plant or a mite feeds. In psyllids, relative density of stomata might be insightful since many species (all?) put their stylets into stomata to access the tissues below. Not sure whether Heteroptera do the same. I would measure something like SLW as a comparative measure and be up front that it may not say much about suitability for actual feeding by either invert.