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In this lesson students view scenes from the Tremors series of movies. Students take notes on the animal’s biology: external anatomy, internal anatomy, lifecycle and behavior. We use our notes to speculate on the evolution and anatomy of these creatures, called Graboids.


Based on the scenes, have the students explain the graboid lifecycle.

Graboid lifecycle

Graboids and sounds

How do graboids navigate underground and detect food sources?

Sonar is the use of sound to navigate, communicate with, or detect objects – on or under the surface of the water – such as another vessel.

Active sonar uses a sound transmitter and a receiver. Active sonar creates a pulse of sound, often called a “ping”, and then listens for reflections (echo) of the pulse. Here we see an animation from the US Navy made in the 1940’s, showing how sonar works.

Old Navy Sub sonar GIF

Some animals have natural sonar, such as bats and whales.

The Tremors movies imply that graboids have a similar way of detecting prey.

Graboid sonar

Internal anatomy

As this is a science fiction movie these creatures aren’t real. But the film makers made it clear that these animals would have realistic internal as well as external anatomy.  In this section we ask students to speculate what kind of organs a creature like this would or wouldn’t have, based on the available information.

Students work in groups to come up with answers – and they have to justify their conclusions. For instance, they might claim that the animal has no skeleton: If so, explain why they conclude this. Or they might claim it does have a skeleton: if so, explain why they conclude this.

graboid anatomy Christopher Stoll dbaubpv

from deviantart.com/christopher-stoll


Evolution of graboids

Based on the observed characteristics, what animals are graboids most closely related to?

What animals in the past might they have evolved from?

Could students make a speculative family tree/cladogram, showing the possible evolution of graboids?

clades & phylogenies

clades rotate = equivalent phylogenies

Gradualism vs. Punctuated Equilibrium

Introductory material

This packet is given to students at the beginning of the class. TBA


Teacher reference material


You may choose to make some of this material available to students during or after viewing the scenes. However, withhold the majority of this material until after the students finish the section in which they speculate and justify their conclusions.


1.0 Introduction

2.0 External Anatomy
2.1 – Graboids
2.2 – Shriekers
2.3 – AssBlasters

3.0 Internal Anatomy

4.0 Ecology

5.0 Evolutionary Overview
5.1 – Evolutionary History
5.2 – Issues of Reproduction
5.3 – Development of Shrieker Legs
5.4 – AssBlasters’ Reproductive Role
5.5 – AssBlaster Biochemistry

6.0 Hypothetical Taxonomy

7.0 Historical and Mythological References

8.0 Threat Assessment

Monster Guide




External anatomy


Internal anatomy




Evolutionary history


The following is from www.scifi.com/tremors/monsters/analysis/06_taxonomy.html

Graboid SyFy

The proper taxonomical classification of Graboids, Shriekers and AssBlasters was a curious challenge because the Graboid species does not clearly belong to any previously known Family grouping. To complete its zoological nomenclature, we were forced to look much deeper into the evolutionary tree than we had expected.

Graboids have been described by some witnesses as being “reptilian,” but this is probably no more accurate than describing the AssBlaster as a bird because it flies or the Shrieker as a frog because it undergoes a metamorphosis. The Graboid does not appear to possess any of the features of true reptiles, though the Shrieker and AssBlaster, curiously, each possess some, such as clawed toes. However, they share just as many similarities with birds and mammals, so a reptilian classification was not indicated.

In fact, Graboids, Shriekers and AssBlasters do not appear to belong to any existing class of vertebrates. They clearly are not fish, and it takes only a slightly more professional observer to see that they they are also neither amphibians nor reptiles, neither birds nor mammals.

It is doubtful that they are even vertebrates, although they do seem to possess endoskeletonlike structures. Vertebrates, it should be stressed, derive from a family of creatures called notochords, which gave rise to fish. Also descended from notochords are amphibians, reptiles, birds and mammals. All these different forms share a heritage of organs and anatomy, ranging from bilateral symmetry to a similarity of organ/tissue types and functions.

The three known forms of genus Caederus lack many of the features inherent to members of the vertebrate line. Most obviously, they lack eyes. Their multistage life cycle is similarly dissociated from known vertebrate reproductive models. In fact, research has not yet yielded any proof that the Graboid species is connected to the vertebrate line.

Regardless, the Graboid, the Shrieker and the AssBlaster are all highly sophisticated lifeforms, which implies that they represent the culmination of a long evolutionary history. Only three other non-vertebrate lines of animal life on Earth have reached a similar level of sophistication: arthropods, annelids and mollusks.

Arthropods (including insects, arachnids, crustaceans and other forms) typically have hard, segmented or jointed exoskeletons, and generally remain small in size when compared with vertebrates. Most arthropods evolved with multiple external limbs and some form of eyes. All these traits are inconsistent with the speculated evolution of C. americana.

Available evidence suggests the Graboid also is not a member of the subphylum Annelida. Annelids — earthworms — share some traits with the Graboid, such as an underground habitat, stiff hairs in the skin to assist in locomotion and an ability to extract nutrients directly from the soil. No annelid, however, has ever possessed anything resembling an endoskeleton or semirigid support system, which C. americana is believed to possess. In addition, C. americana and C. mexicana possess other features not found in annelids: segmented jaws; prehensile mouth tentacles; a multiphase life cycle; and thermal sensors. The Graboid is also larger and more sophisticated than any known annelid, making it highly unlikely that genus Caederus belongs in this subphylum.

Genus Caederus might be unique, in a class of its own. It might even be extraterrestrial. More likely, though, it is a form of mollusk.

The subphylum Mollusca is one of the oldest, most diversified and successful on Earth. It includes clams, mussels, snails, slugs, cuttlefish, nautili, squids and octopi. The most advanced form of mollusks are the cephalopods (octopi and squids), which share many important features with the Graboid.

Cephalopods have multiple tentacles, ranging from eight to dozens, all surrounding a mouth or gullet — an arrangement that resembles the Graboid’s tentacled mouth structure. Furthermore, some cephalopods (such as the prehistoric ammonites or the modern nautilus) have external shells or carapaces, as does the Graboid.

At least one cephalopod, the cuttlefish, has a Graboid-like external carapace, or bony structure. In addition, octopi have enough control over the muscles of their skin to change their texture from craggy to smooth, suggesting a skin musculature similar to that of the Graboid, although of different degree.

The “wing structure” of the AssBlaster bears at least a passing resemblance to the rippling “fins” of the cuttlefish. Although no known aquatic cephalopod ejects combustible compounds, it is a compelling similarity that several eject prodigious clouds of ink as a defensive mechanism, and some have a hydrojet-like propulsive organ that resembles the AssBlaster’s dramatically fiery self-launching ability.

Cephalopods are water-breathers, but other mollusks, including snails and slugs, exist on dry land. Many cephalopods, as well as certain bivalve mollusks, are able to survive for short durations out of the water.

Cephalopods are the most intelligent non-vertebrate animals known to exist. Studies have indicated that they might possess a capacity for memory, learning and problem-solving, and witnesses have reported signs of social behavior among groups of squid and octopi. Cephalopods might well be as intelligent as some species of birds or mammals; certainly, they seem to show a level of “smart” behavior similar to that of genus Caederus.

Finally, cephalopods have managed to achieve significant size and mass in aquatic habitats. The giant squid, for instance, is a deep-ocean-dweller that might rival the Graboid in size. The largest known giant squid have weighed several tons and stretched up to 55 feet from their flukes to the extremity of their longest tentacle.

Although the Graboid and its related forms possess features previously undocumented among cephalopods (such as jointed limbs, endoskeletons and a multiphase life cycle), these differences do not disqualify their categorization as mollusks. For example, bivalve mollusks (clams and mussels) possess hinged shells; it is not unreasonable to assume that the Graboid family of mollusks may have developed hinged internal shells and eventually evolved more complex internal skeletons.

However, no mollusk has evolved anything resembling the thermal sensors of the Shrieker and AssBlaster; likewise, the incendiary metabolism of the AssBlaster is unique to the Graboid species. Furthermore, no cephalopod or other mollusk possesses a life cycle nearly as complex as that of genus Caederus.

Still, the shared traits documented above and elsewhere in this document are significant enough to justify a tentative classification of the Graboid, the Shrieker and the AssBlaster as distant, terrestrial relatives of class Cephalopoda.

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Learning Standards

This unit addresses critical thinking skills in the Next Generation Science Standards, which are based on “A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas”, by the National Research Council of the National Academies. In this document we read

“Through discussion and reflection, students can come to realize that scientific inquiry embodies a set of values. These values include respect for the importance of logical thinking, precision, open-mindedness, objectivity, skepticism, and a requirement for transparent research procedures and honest reporting of findings.”

Next Generation Science Standards: Science & Engineering Practices
● Ask questions that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
● Ask questions that arise from examining models or a theory, to clarify and/or seek additional information and relationships.
● Ask questions to determine relationships, including quantitative relationships, between independent and dependent variables.
● Ask questions to clarify and refine a model, an explanation, or an engineering problem.
● Evaluate a question to determine if it is testable and relevant.
● Ask questions that can be investigated within the scope of the school laboratory, research facilities, or field (e.g., outdoor environment) with available resources and, when appropriate, frame a hypothesis based on a model or theory.
● Ask and/or evaluate questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of the design

Science and engineering practices: NSTA National Science Teacher Association

Next Gen Science Standards Appendix F: Science and engineering practices

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