INTRODUCTION. 



Xlll 



bidse, and Dytiscidae, slender, and usually divided into two joints 

 like a palpus, whence in the older books the insects of those 

 families are said to have six palpi. The inner lobe is, by a rare 

 exception, as in most Cicindelidse, terminated by a movable hook. 

 The maxillary palpi are usually 4-jointed, rarely 3-jointed, and 

 in the genus Aleochara alone 5-jointed, by the addition of a very 

 minute terminal piece ; they vary in form, being filiform or dilated, 

 and are occasionally of great size, as in most PselapMdse ; some- 

 times very long and slender, as in most Hydrophilidse ; the last 

 joint is frequently much smaller and more slender than the penul- 

 timate, in which case the palpi are called subulate. 



Axtenn;e: 1. Bipectinate ; 2. Flabellate. Maxilla: 3. Bembidiura ; 4. Hydrophilus ; 

 5. Pselaphus. Maxillary palpi : 6. Ctenistes; 7. Tmesiphorus ; 8. Tychus. 



Mentum and Ligula. — Beneath the maxillae, and between 

 them, is seen a small movable piece called the ligula, which sup- 

 ports two articulated appendages called the labial palpi; behind 

 them is a large or small corneous plate, forming the floor of the 

 mouth — it is called the mentum,, and articulates with the lower 

 surface of the head by the gular suture, which is rarely effaced ; 

 the openings each side of the mentum are called buccal ofjenings. 

 The mentum and ligula are very important organs in the system 

 of classification. 



The mentum is usually small, or moderate in size, and trape- 

 zoidal or quadrate, but frequently of large size, and varies accord- 



