A crawler undercarriage configured for modular attachment of work tools wherein the crawler undercarriage operates on at least two endless crawler tracks. The pair of transparent substrates are opposed to each other with a distance therebetween. The cover plate portion includes first and second faces. At least one of the mirrors has a central aperture containing the optical axis. Tags may be suspended in a cantilevered manner during marking. Each heater element is configured such that the energy required to be applied thereto to cause ejection of the drop is less than the energy required to heat a volume of the ejectable liquid equal to the volume of the drop to a temperature less than 40 degrees Celsius above ambient temperature.